System.Data.SQLite

        if (_sql == null)
            throw new InvalidOperationException(
                "Database connection not valid for binding functions.");

        _sql.BindFunction(functionAttribute, function, _flags);
    }

    ///////////////////////////////////////////////////////////////////////////////////////////////

    /// <summary>
    /// Attempts to bind the specified <see cref="SQLiteFunction" /> object
    /// instance to this connection.
    /// </summary>
    /// <param name="functionAttribute">
    /// The <see cref="SQLiteFunctionAttribute" /> object instance containing
    /// the metadata for the function to be bound.
    /// </param>
    /// <param name="callback">
    /// The <see cref="Delegate" /> object instance that implements the
    /// function to be bound.
    /// </param>
    public void BindFunction(
        SQLiteFunctionAttribute functionAttribute,
        Delegate callback
        )
    {
        CheckDisposed();

        if (_sql == null)
            throw new InvalidOperationException(
                "Database connection not valid for binding functions.");

        _sql.BindFunction(functionAttribute,
            new SQLiteDelegateFunction(callback), _flags);
    }

    ///////////////////////////////////////////////////////////////////////////////////////////////

    /// <summary>
    /// Attempts to unbind the specified <see cref="SQLiteFunction" /> object
    /// instance to this connection.
    /// </summary>

    /// </summary>
    private SQLiteCallback  _StepFunc;
    /// <summary>
    /// Holds a reference to the callback function for finalizing an aggregate function
    /// </summary>
    private SQLiteFinalCallback  _FinalFunc;
    /// <summary>
    /// Holds a reference to the callback function for collating sequences
    /// </summary>
    private SQLiteCollation _CompareFunc;

    private SQLiteCollation _CompareFunc16;

    /// <summary>
    /// Current context of the current callback.  Only valid during a callback

    public virtual object Final(object contextData)
    {
      CheckDisposed();
      return null;
    }

    /// <summary>
    /// User-defined collating sequences override this method to provide a custom string sorting algorithm.
    /// </summary>
    /// <param name="param1">The first string to compare</param>
    /// <param name="param2">The second strnig to compare</param>
    /// <returns>1 if param1 is greater than param2, 0 if they are equal, or -1 if param1 is less than param2</returns>
    public virtual int Compare(string param1, string param2)
    {
      CheckDisposed();

            {
                // do nothing.
            }
        }
    }

    /// <summary>
    /// Internal collating sequence function, which wraps up the raw string pointers and executes the Compare() virtual function.
    /// WARNING: Must not throw exceptions.
    /// </summary>
    /// <param name="ptr">Not used</param>
    /// <param name="len1">Length of the string pv1</param>
    /// <param name="ptr1">Pointer to the first string to compare</param>
    /// <param name="len2">Length of the string pv2</param>
    /// <param name="ptr2">Pointer to the second string to compare</param>

        if ((_base != null) && _base.IsOpen())
            _base.Cancel();

        return 0;
    }

    /// <summary>
    /// Internal collating sequence function, which wraps up the raw string pointers and executes the Compare() virtual function.
    /// WARNING: Must not throw exceptions.
    /// </summary>
    /// <param name="ptr">Not used</param>
    /// <param name="len1">Length of the string pv1</param>
    /// <param name="ptr1">Pointer to the first string to compare</param>
    /// <param name="len2">Length of the string pv2</param>
    /// <param name="ptr2">Pointer to the second string to compare</param>

    /// <summary>
    /// Manual method of registering a function.  The type must still have the SQLiteFunctionAttributes in order to work
    /// properly, but this is a workaround for the Compact Framework where enumerating assemblies is not currently supported.
    /// </summary>
    /// <param name="typ">The type of the function to register</param>
    public static void RegisterFunction(Type typ)
    {
        object[] arAtt = typ.GetCustomAttributes(

            typeof(SQLiteFunctionAttribute), false);

        for (int y = 0; y < arAtt.Length; y++)
      {
            SQLiteFunctionAttribute at = arAtt[y] as SQLiteFunctionAttribute;

            if (at == null)
                continue;

            RegisterFunction(
                at.Name, at.Arguments, at.FuncType, at.InstanceType,
                at.Callback);
        }
    }

    /// <summary>
    /// Alternative method of registering a function.  This method
    /// does not require the specified type to be annotated with
    /// <see cref="SQLiteFunctionAttribute" />.
    /// </summary>
    /// <param name="name">
    /// The name of the function to register.
    /// </param>
    /// <param name="argumentCount">
    /// The number of arguments accepted by the function.
    /// </param>
    /// <param name="functionType">
    /// The type of SQLite function being resitered (e.g. scalar,
    /// aggregate, or collating sequence).
    /// </param>
    /// <param name="instanceType">
    /// The <see cref="Type" /> that actually implements the function.
    /// This will only be used if the <paramref name="callback" />
    /// parameter is null.
    /// </param>
    /// <param name="callback">
    /// The <see cref="Delegate" /> that implements the function.  If
    /// this is non-null, the <paramref name="instanceType" /> parameter
    /// will be ignored when the function is invoked.
    /// </param>
    public static void RegisterFunction(
        string name,
        int argumentCount,
        FunctionType functionType,
        Type instanceType,
        Delegate callback
        )
    {
        SQLiteFunctionAttribute at = new SQLiteFunctionAttribute(
            name, argumentCount, functionType);

        at.InstanceType = instanceType;
        at.Callback = callback;

          _registeredFunctions.Add(at, null);
        }

    /// <summary>
    /// Creates a <see cref="SQLiteFunction" /> instance based on the specified
    /// <see cref="SQLiteFunctionAttribute" />.
    /// </summary>
    /// <param name="functionAttribute">
    /// The <see cref="SQLiteFunctionAttribute" /> containing the metadata about
    /// the function to create.
    /// </param>
    /// <param name="function">
    /// The created function -OR- null if the function could not be created.
    /// </param>
    /// <returns>
    /// Non-zero if the function was created; otherwise, zero.
    /// </returns>
    private static bool CreateFunction(
        SQLiteFunctionAttribute functionAttribute,
        out SQLiteFunction function
        )
    {
        if (functionAttribute == null)
        {
            function = null;
            return false;
        }
        else if (functionAttribute.Callback != null)
        {
            function = new SQLiteDelegateFunction(
                functionAttribute.Callback);

            return true;
        }
        else if (functionAttribute.InstanceType != null)
        {
            function = (SQLiteFunction)Activator.CreateInstance(
                functionAttribute.InstanceType);

            return true;
        }
        else
        {
            function = null;
            return false;
      }
    }

    /// <summary>
    /// Called by the SQLiteBase derived classes, this method binds all registered (known) user-defined functions to a connection.
    /// It is done this way so that all user-defined functions will access the database using the same encoding scheme
    /// as the connection (UTF-8 or UTF-16).

                in _registeredFunctions)
        {
            SQLiteFunctionAttribute pr = pair.Key;

            if (pr == null)
                continue;

            SQLiteFunction f;

            if (CreateFunction(pr, out f))
            {
            BindFunction(sqlbase, pr, f, flags);
            lFunctions[pr] = f;
        }
            else
            {
                lFunctions[pr] = null;
            }
        }

        return lFunctions;
    }

    /// <summary>
    /// Called by the SQLiteBase derived classes, this method unbinds all registered (known)
    /// functions -OR- all previously bound user-defined functions from a connection.

            }
        }

        return result;
    }

    /// <summary>
    /// This function binds a user-defined function to a connection.
    /// </summary>
    /// <param name="sqliteBase">
    /// The <see cref="SQLiteBase" /> object instance associated with the
    /// <see cref="SQLiteConnection" /> that the function should be bound to.
    /// </param>
    /// <param name="functionAttribute">
    /// The <see cref="SQLiteFunctionAttribute"/> object instance containing

                name, null, null, false) == SQLiteErrorCode.Ok;
        }
    }
  }



  /////////////////////////////////////////////////////////////////////////////

  /// <summary>
  /// This class implements a SQLite function using a <see cref="Delegate" />.
  /// All the virtual methods of the <see cref="SQLiteFunction" /> class are
  /// implemented using calls to the <see cref="Delegate.DynamicInvoke" />
  /// method.  The arguments are presented in the same order they appear in
  /// the associated <see cref="SQLiteFunction" /> methods with one exception:
  /// the first argument is the name of the virtual method being implemented.
  /// </summary>
  public class SQLiteDelegateFunction : SQLiteFunction
  {
      #region Private Constants
      /// <summary>
      /// This error message is used by the overridden virtual methods when the
      /// callback has not been set.
      /// </summary>
      private const string NoCallbackError = "No callback is set.";

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// This error message is used by the overridden <see cref="Compare" />
      /// method when the result does not have a type of <see cref="Int32" />.
      /// </summary>
      private const string ResultInt32Error = "\"{0}\" result must be Int32.";
      #endregion

      /////////////////////////////////////////////////////////////////////////

      #region Public Constructors
      /// <summary>
      /// Constructs an empty instance of this class.
      /// </summary>
      public SQLiteDelegateFunction()
          : this(null)
      {
          // do nothing.
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// Constructs an instance of this class using the specified
      /// <see cref="Delegate" /> as the <see cref="SQLiteFunction" />
      /// implementation.
      /// </summary>
      /// <param name="callback">
      /// The <see cref="Delegate" /> to be used for all calls into the
      /// virtual methods needed by the <see cref="SQLiteFunction" />
      /// class.
      /// </param>
      public SQLiteDelegateFunction(
          Delegate callback
          )
      {
          this.callback = callback;
      }
      #endregion

      /////////////////////////////////////////////////////////////////////////

      #region Protected Methods
      /// <summary>
      /// Returns the list of arguments for the <see cref="Invoke" /> method,
      /// as an <see cref="Array" /> of <see cref="Object" />.  The first
      /// argument is always the literal string "Invoke".
      /// </summary>
      /// <param name="args">
      /// The original arguments received by the <see cref="Invoke" /> method.
      /// </param>
      /// <returns>
      /// The arguments to pass to the configured <see cref="Delegate" />.
      /// </returns>
      protected virtual object[] GetInvokeArgs(
          object[] args
          ) /* CANNOT RETURN NULL */
      {
          if (args == null)
              return new object[] { "Invoke" };

          object[] newArgs = new object[args.Length + 1];

          newArgs[0] = "Invoke";

          for (int index = 0; index < args.Length; index++)
              newArgs[index + 1] = args[index];

          return newArgs;
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// Returns the list of arguments for the <see cref="Step" /> method,
      /// as an <see cref="Array" /> of <see cref="Object" />.  The first
      /// argument is always the literal string "Step".
      /// </summary>
      /// <param name="args">
      /// The original arguments received by the <see cref="Step" /> method.
      /// </param>
      /// <param name="stepNumber">
      /// The step number (one based).  This is incrememted each time the
      /// <see cref="Step" /> method is called.
      /// </param>
      /// <param name="contextData">
      /// A placeholder for implementers to store contextual data pertaining
      /// to the current context.
      /// </param>
      /// <returns>
      /// The arguments to pass to the configured <see cref="Delegate" />.
      /// </returns>
      protected virtual object[] GetStepArgs(
          object[] args,
          int stepNumber,
          object contextData
          ) /* CANNOT RETURN NULL */
      {
          int newLength = 3; /* "Step", stepNumber, contextData */

          if (args != null)
              newLength += args.Length;

          object[] newArgs = new object[newLength];

          newArgs[0] = "Step";

          if (args != null)
              for (int index = 0; index < args.Length; index++)
                  newArgs[index + 1] = args[index];

          newArgs[newLength - 2] = stepNumber;
          newArgs[newLength - 1] = contextData;

          return newArgs;
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// Returns the list of arguments for the <see cref="Final" /> method,
      /// as an <see cref="Array" /> of <see cref="Object" />.  The first
      /// argument is always the literal string "Final".
      /// </summary>
      /// <param name="contextData">
      /// A placeholder for implementers to store contextual data pertaining
      /// to the current context.
      /// </param>
      /// <returns>
      /// The arguments to pass to the configured <see cref="Delegate" />.
      /// </returns>
      protected virtual object[] GetFinalArgs(
          object contextData
          ) /* CANNOT RETURN NULL */
      {
          return new object[] { "Final", contextData };
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// Returns the list of arguments for the <see cref="Compare" /> method,
      /// as an <see cref="Array" /> of <see cref="Object" />.  The first
      /// argument is always the literal string "Compare".
      /// </summary>
      /// <param name="param1">
      /// The first string to compare.
      /// </param>
      /// <param name="param2">
      /// The second strnig to compare.
      /// </param>
      /// <returns>
      /// The arguments to pass to the configured <see cref="Delegate" />.
      /// </returns>
      protected virtual object[] GetCompareArgs(
          string param1,
          string param2
          ) /* CANNOT RETURN NULL */
      {
          return new object[] { "Compare", param1, param2 };
      }
      #endregion

      /////////////////////////////////////////////////////////////////////////

      #region Public Properties
      private Delegate callback;
      /// <summary>
      /// The <see cref="Delegate" /> to be used for all calls into the
      /// virtual methods needed by the <see cref="SQLiteFunction" />
      /// class.
      /// </summary>
      public virtual Delegate Callback
      {
          get { return callback; }
          set { callback = value; }
      }
      #endregion

      /////////////////////////////////////////////////////////////////////////

      #region System.Data.SQLite.SQLiteFunction Overrides
      /// <summary>
      /// This virtual method is the implementation for scalar functions.
      /// See the <see cref="SQLiteFunction.Invoke" /> method for more
      /// details.
      /// </summary>
      /// <param name="args">
      /// The arguments for the scalar function.  The first argument is always
      /// the literal string "Invoke".  The remaining arguments, if any, are
      /// passed exactly as they are received.
      /// </param>
      /// <returns>
      /// The result of the scalar function.
      /// </returns>
      public override object Invoke(
          object[] args /* in */
          )
      {
          if (callback == null)
              throw new InvalidOperationException(NoCallbackError);

          return callback.DynamicInvoke(GetInvokeArgs(args)); /* throw */
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// This virtual method is part of the implementation for aggregate
      /// functions.  See the <see cref="SQLiteFunction.Step" /> method
      /// for more details.
      /// </summary>
      /// <param name="args">
      /// The arguments for the aggregate function.  The first argument is
      /// always the literal string "Step".  The remaining arguments, if
      /// any, are passed exactly as they are received.
      /// </param>
      /// <param name="stepNumber">
      /// The step number (one based).  This is incrememted each time the
      /// <see cref="Step" /> method is called.
      /// </param>
      /// <param name="contextData">
      /// A placeholder for implementers to store contextual data pertaining
      /// to the current context.
      /// </param>
      public override void Step(
          object[] args,         /* in */
          int stepNumber,        /* in */
          ref object contextData /* in, out */
          )
      {
          if (callback == null)
              throw new InvalidOperationException(NoCallbackError);

          object[] newArgs = GetStepArgs(args, stepNumber, contextData);

          /* IGNORED */
          callback.DynamicInvoke(newArgs); /* throw */

          contextData = newArgs[newArgs.Length - 1]; /* out */
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// This virtual method is part of the implementation for aggregate
      /// functions.  See the <see cref="SQLiteFunction.Final" /> method
      /// for more details.
      /// </summary>
      /// <param name="contextData">
      /// A placeholder for implementers to store contextual data pertaining
      /// to the current context.
      /// </param>
      /// <returns>
      /// The result of the aggregate function.
      /// </returns>
      public override object Final(
          object contextData /* in */
          )
      {
          if (callback == null)
              throw new InvalidOperationException(NoCallbackError);

          return callback.DynamicInvoke(GetFinalArgs(contextData)); /* throw */
      }

      /////////////////////////////////////////////////////////////////////////

      /// <summary>
      /// This virtual method is part of the implementation for collating
      /// sequences.  See the <see cref="SQLiteFunction.Compare" /> method
      /// for more details.
      /// </summary>
      /// <param name="param1">
      /// The first string to compare.
      /// </param>
      /// <param name="param2">
      /// The second strnig to compare.
      /// </param>
      /// <returns>
      /// A positive integer if the <paramref name="param1" /> parameter is
      /// greater than the <paramref name="param2" /> parameter, a negative
      /// integer if the <paramref name="param1" /> parameter is less than
      /// the <paramref name="param2" /> parameter, or zero if they are
      /// equal.
      /// </returns>
      public override int Compare(
          string param1, /* in */
          string param2  /* in */
          )
      {
          if (callback == null)
              throw new InvalidOperationException(NoCallbackError);

          object[] newArgs = GetCompareArgs(param1, param2);
          object result = callback.DynamicInvoke(newArgs); /* throw */

          if (result is int)
              return (int)result;

          throw new InvalidOperationException(String.Format(
              ResultInt32Error, newArgs[0]));
      }
      #endregion
  }

  /////////////////////////////////////////////////////////////////////////////

  /// <summary>
  /// Extends SQLiteFunction and allows an inherited class to obtain the collating sequence associated with a function call.
  /// </summary>
  /// <remarks>
  /// User-defined functions can call the GetCollationSequence() method in this class and use it to compare strings and char arrays.
  /// </remarks>

    Scalar = 0,
    /// <summary>
    /// Aggregate functions are designed to accumulate data until the end of a call and then return a result gleaned from the accumulated data.
    /// Examples include SUM(), COUNT(), AVG(), etc.
    /// </summary>
    Aggregate = 1,
    /// <summary>
    /// Collating sequences are used to sort textual data in a custom manner, and appear in an ORDER BY clause.  Typically text in an ORDER BY is
    /// sorted using a straight case-insensitive comparison function.  Custom collating sequences can be used to alter the behavior of text sorting
    /// in a user-defined manner.
    /// </summary>
    Collation = 2,
  }

  /// <summary>

  /// </summary>
  /// <param name="context">Raw context pointer for the user function</param>
#if !PLATFORM_COMPACTFRAMEWORK
  [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
#endif
  internal delegate void SQLiteFinalCallback(IntPtr context);
  /// <summary>
  /// Internal callback delegate for implementing collating sequences
  /// </summary>
  /// <param name="puser">Not used</param>
  /// <param name="len1">Length of the string pv1</param>
  /// <param name="pv1">Pointer to the first string to compare</param>
  /// <param name="len2">Length of the string pv2</param>
  /// <param name="pv2">Pointer to the second string to compare</param>
  /// <returns>Returns -1 if the first string is less than the second.  0 if they are equal, or 1 if the first string is greater

  [AttributeUsage(AttributeTargets.Class, Inherited = false, AllowMultiple = true)]
  public sealed class SQLiteFunctionAttribute : Attribute
  {
    private string       _name;
    private int          _argumentCount;
    private FunctionType _functionType;
    private Type         _instanceType;
    private Delegate     _callback;

    /// <summary>
    /// Default constructor, initializes the internal variables for the function.
    /// </summary>
    public SQLiteFunctionAttribute()
        : this(String.Empty, -1, FunctionType.Scalar)
    {

        FunctionType functionType
        )
    {
        _name = name;
        _argumentCount = argumentCount;
        _functionType = functionType;
        _instanceType = null;
        _callback = null;
    }

    /// <summary>
    /// The function's name as it will be used in SQLite command text.
    /// </summary>
    public string Name
    {

    {
      get { return _functionType; }
      set { _functionType = value; }
    }

    /// <summary>
    /// The <see cref="System.Type" /> object instance that describes the class
    /// containing the implementation for the associated function.  The value of
    /// this property will not be used if the <see cref="Callback" /> property
    /// value is set to non-null.
    /// </summary>
    internal Type InstanceType
    {
        get { return _instanceType; }
        set { _instanceType = value; }
    }

    /// <summary>
    /// The <see cref="Delegate" /> that refers to the implementation for the
    /// associated function.  If this property value is set to non-null, it will
    /// be used instead of the <see cref="InstanceType" /> property value.
    /// </summary>
    internal Delegate Callback
    {
        get { return _callback; }
        set { _callback = value; }
}
  }
}