public event QueryCustomFunctionsEventHandler QueryCustomFunctions

Public Event QueryCustomFunctions As QueryCustomFunctionsEventHandler

using DevExpress.Data.Filtering;

IsBlackFridayDiscountFunction.Register();
gridView1.QueryCustomFunctions += OnQueryCustomFunctions;

void OnQueryCustomFunctions(object sender, CustomFunctionEventArgs e) {
    if(e.PropertyName == "Discount")
        e.Add(IsBlackFridayDiscountFunction.FunctionName);
}

public class IsBlackFridayDiscountFunction : ICustomFunctionDisplayAttributes {
    public const string FunctionName = "IsBlackFridayDiscount";
    static readonly IsBlackFridayDiscountFunction Instance = new IsBlackFridayDiscountFunction();
    IsBlackFridayDiscountFunction() { }
    public static void Register() {
        CriteriaOperator.RegisterCustomFunction(Instance);
    }
    public static bool Unregister() {
        return CriteriaOperator.UnregisterCustomFunction(Instance);
        }
    #region ICustomFunctionOperatorBrowsable Members
    public FunctionCategory Category {
        get { return FunctionCategory.Math; }
    }
    public string Description {
        get { return "The discount amount is 15% or more."; }
    }
    public bool IsValidOperandCount(int count) {
        return count == 1;
    }
    public bool IsValidOperandType(int operandIndex, int operandCount, Type type) {
        return DevExpress.Data.Summary.SummaryItemTypeHelper.IsNumericalType(type);
    }
    public int MaxOperandCount {
        get { return 1; }
    }
    public int MinOperandCount {
        get { return 1; }
    }
    #endregion
    #region ICustomFunctionDisplayAttributes
    public string DisplayName {
        get { return "Is Black Friday Discount"; }
    }
    public object Image {
        get { return "bo_price"; }
    }
    #endregion
    #region ICustomFunctionOperator Members
    public object Evaluate(params object[] operands) {
        double discount = Convert.ToDouble(operands[0]);
        return discount >= 0.15;
    }
    public string Name {
        get { return FunctionName; }
    }
    public Type ResultType(params Type[] operands) {
        return typeof(bool);
   }
    #endregion
}

Imports DevExpress.Data.Filtering

IsBlackFridayDiscountFunction.Register()
AddHandler gridView1.QueryCustomFunctions, AddressOf OnQueryCustomFunctions

Private Sub OnQueryCustomFunctions(ByVal sender As Object, ByVal e As Data.Filtering.CustomFunctionEventArgs)
    If e.PropertyName = "Discount" Then
        e.Add(IsBlackFridayDiscountFunction.FunctionName)
    End If
End Sub

Public Class IsBlackFridayDiscountFunction
    Implements ICustomFunctionDisplayAttributes

    Public Const FunctionName As String = "IsBlackFridayDiscount"
    Private Shared ReadOnly Instance As New IsBlackFridayDiscountFunction()
    Private Sub New()
    End Sub
        '
    Public Shared Sub Register()
        CriteriaOperator.RegisterCustomFunction(Instance)
    End Sub
    Public Shared Function Unregister() As Boolean
        Return CriteriaOperator.UnregisterCustomFunction(Instance)
    End Function
    #Region "ICustomFunctionOperatorBrowsable Members"
    Public ReadOnly Property Category() As FunctionCategory Implements DevExpress.Data.Filtering.ICustomFunctionOperatorBrowsable.Category
        Get
            Return FunctionCategory.Math
        End Get
    End Property
    Public ReadOnly Property Description() As String Implements DevExpress.Data.Filtering.ICustomFunctionOperatorBrowsable.Description
        Get
            Return "The discount amount is 15% or more."
        End Get
    End Property
    Public Function IsValidOperandCount(ByVal count As Integer) As Boolean Implements DevExpress.Data.Filtering.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 DevExpress.Data.Filtering.ICustomFunctionOperatorBrowsable.IsValidOperandType
        Return DevExpress.Data.Summary.SummaryItemTypeHelper.IsNumericalType(type)
    End Function
    Public ReadOnly Property MaxOperandCount() As Integer Implements DevExpress.Data.Filtering.ICustomFunctionOperatorBrowsable.MaxOperandCount
        Get
            Return 1
        End Get
    End Property
    Public ReadOnly Property MinOperandCount() As Integer Implements DevExpress.Data.Filtering.ICustomFunctionOperatorBrowsable.MinOperandCount
        Get
            Return 1
        End Get
    End Property
    #End Region
    #Region "ICustomFunctionDisplayAttributes"
    Public ReadOnly Property DisplayName() As String Implements ICustomFunctionDisplayAttributes.DisplayName
        Get
            Return "Is Black Friday Discount"
        End Get
    End Property
    Public ReadOnly Property Image() As Object Implements ICustomFunctionDisplayAttributes.Image
        Get
            Return "bo_price"
        End Get
    End Property
    #End Region
    #Region "ICustomFunctionOperator Members"
    Public Function Evaluate(ParamArray ByVal operands() As Object) As Object Implements DevExpress.Data.Filtering.ICustomFunctionOperator.Evaluate
        Dim discount As Double = Convert.ToDouble(operands(0))
        Return discount >= 0.15
    End Function
    Public ReadOnly Property Name() As String Implements DevExpress.Data.Filtering.ICustomFunctionOperator.Name
        Get
            Return FunctionName
        End Get
    End Property
    Public Function ResultType(ParamArray ByVal operands() As Type) As Type Implements DevExpress.Data.Filtering.ICustomFunctionOperator.ResultType
        Return GetType(Boolean)
    End Function
    #End Region
End Class

[CustomFunction(IsBlackFridayDiscountFunction.FunctionName)]
public double Discount { get; set; }

<CustomFunction(IsBlackFridayDiscountFunction.FunctionName)>
Public Property Discount() As Double