ICustomFunctionOperatorFormattable.Format(Type, String[]) Method

Returns a string for insertion into a SQL query when the custom function is evaluated (before a query is sent to the data source).

Namespace: DevExpress.Data.Filtering

Assembly: DevExpress.Data.v23.1.dll

NuGet Package: DevExpress.Data

Declaration

C#

string Format(
    Type providerType,
    params string[] operands
)

VB.NET

Function Format(
    providerType As Type,
    ParamArray operands As String()
) As String

Parameters

Name	Type	Description
providerType	Type	A Type object that identifies the data provider. If the data source of the query expression is Microsoft SQL Server, the type is MSSqlConnectionProvider.
operands	String[]	An array of operands (parameters) passed to a custom function.

Returns

Type	Description
String	A String that substitutes a call to the custom function in a query statement.

Remarks

You can follow these steps to implement the Format method:

1. Find a suitable SQL command or syntax.
2. Return a dummy value (operands[0]) to the Format function.
3. Set a breakpoint at the Format method to inspect what values it receives.
4. Continue execution to log the SQL command.
5. Generate the string you would insert in this command to get the desired result.
6. Implement the Format method so that it returns the desired string based on the current data provider.

Example

The following code implements the StdDev custom function intended only for SQL queries in Query Builder. The Format method modifies the query string.

When this function is used in an expression, the Evaluate method throws an exception.

C#

using System;
using System.Collections.Generic;
using DevExpress.Data.Filtering;


namespace SelectQueryWindowsFormsApplication {
    public sealed class StDevFunction : ICustomFunctionOperatorBrowsable, ICustomFunctionOperatorFormattable {
        const string name = "StDev";
        public static void Register() {
            CriteriaOperator.RegisterCustomFunction(new StDevFunction());
        }
        public static void Unregister() {
            CriteriaOperator.UnregisterCustomFunction(name);
        }
        public static readonly HashSet<Type> ValidOperandTypes = new HashSet<Type> {
            typeof(sbyte),
            typeof(byte),
            typeof(short),
            typeof(ushort),
            typeof(int),
            typeof(uint),
            typeof(long),
            typeof(ulong),
            typeof(decimal),
            typeof(double),
            typeof(float)
        };

        #region ICustomFunctionOperator
        public Type ResultType(params Type[] operands) {
            return typeof(double);
        }
        public object Evaluate(params object[] operands) {
            throw new NotSupportedException();
        }
        public string Name { get { return name; } }
        #endregion

        #region ICustomFunctionOperatorBrowsable
        public FunctionCategory Category { get { return FunctionCategory.Math; } }
        public string Description { get { return "Standard deviation function"; } }
        public bool IsValidOperandCount(int count) {
            return count == 1;
        }
        public bool IsValidOperandType(int operandIndex, int operandCount, Type type) {
            return ValidOperandTypes.Contains(type);
        }
        public int MaxOperandCount { get { return 1; } }
        public int MinOperandCount { get { return 1; } }
        #endregion

        #region ICustomFunctionOperatorFormattable
        public string Format(Type providerType, params string[] operands) {
            return string.Format("stdev({0})", operands[0]);
        }
        #endregion
    }
}

VB.NET

Imports System
Imports System.Collections.Generic
Imports DevExpress.Data.Filtering


Namespace SelectQueryWindowsFormsApplication
    Public NotInheritable Class StDevOperator
        Implements ICustomFunctionOperatorBrowsable, ICustomFunctionOperatorFormattable
        Private Const name_Renamed As String = "StDev"
        Public Shared Sub Register()
            CriteriaOperator.RegisterCustomFunction(New StDevOperator())
        End Sub
        Public Shared Sub Unregister()
            CriteriaOperator.UnregisterCustomFunction(name_Renamed)
        End Sub
        Public Shared ReadOnly ValidOperandTypes As New HashSet(Of Type)() From {GetType(SByte), GetType(Byte), GetType(Short), GetType(UShort), GetType(Integer), GetType(UInteger), GetType(Long), GetType(ULong), GetType(Decimal), GetType(Double), GetType(Single)}

#Region "ICustomFunctionOperator"
        Public Function ResultType(ParamArray ByVal operands() As Type) As Type Implements ICustomFunctionOperator.ResultType
            Return GetType(Double)
        End Function
        Public Function Evaluate(ParamArray ByVal operands() As Object) As Object Implements ICustomFunctionOperator.Evaluate
            Throw New NotSupportedException()
        End Function
        Public ReadOnly Property Name() As String Implements ICustomFunctionOperator.Name
            Get
                Return name_Renamed
            End Get
        End Property

#End Region
#Region "ICustomFunctionOperatorBrowsable"
        Public ReadOnly Property Category() As FunctionCategory Implements ICustomFunctionOperatorBrowsable.Category
            Get
                Return FunctionCategory.Math
            End Get
        End Property
        Public ReadOnly Property Description() As String Implements ICustomFunctionOperatorBrowsable.Description
            Get
                Return "Standard deviation function"
            End Get
        End Property
        Public Function IsValidOperandCount(ByVal count As Integer) As Boolean Implements ICustomFunctionOperatorBrowsable.IsValidOperandCount
            Return count = 1
        End Function
        Public Function IsValidOperandType(ByVal operandIndex As Integer, ByVal operandCount As Integer, ByVal type As Type) As Boolean Implements ICustomFunctionOperatorBrowsable.IsValidOperandType
            Return ValidOperandTypes.Contains(type)
        End Function
        Public ReadOnly Property MaxOperandCount() As Integer Implements ICustomFunctionOperatorBrowsable.MaxOperandCount
            Get
                Return 1
            End Get
        End Property
        Public ReadOnly Property MinOperandCount() As Integer Implements ICustomFunctionOperatorBrowsable.MinOperandCount
            Get
                Return 1
            End Get
        End Property
#End Region

#Region "ICustomFunctionOperatorFormattable"
        Public Function Format(ByVal providerType As Type, ParamArray ByVal operands() As String) As String Implements ICustomFunctionOperatorFormattable.Format
            Return String.Format("stdev({0})", operands(0))
        End Function
#End Region
    End Class
End Namespace

View Example: Reporting for WinForms - Implement a Custom Function for Use in a Query Expression