using System;
using DevExpress.Data.Filtering;
using DevExpress.Xpo.DB;

// ...
public class MyGetMonthFunction : ICustomFunctionOperatorFormattable {
    #region ICustomFunctionOperatorFormattable Members
    // The function's expression to be evaluated on the server.
    string ICustomFunctionOperatorFormattable.Format(Type providerType, params string[] operands) {
        // This example implements the function for Microsoft Access databases only.
        if (providerType == typeof(AccessConnectionProvider))
            return string.Format("datepart(\"m\", {0})", operands[0]);
        throw new NotSupportedException(string.Concat(
            "This provider is not supported: ", providerType.Name));
    }

    public static int MyGetMonth(DateTime date) {
        return date.Month;
    }
    #endregion

    #region ICustomFunctionOperator Members
    // Evaluates the function on the client.
    object ICustomFunctionOperator.Evaluate(params object[] operands) {
        return MyGetMonth((DateTime)operands[0]);
    }

    string ICustomFunctionOperator.Name {
        get { return nameof(MyGetMonth); }
    }

    Type ICustomFunctionOperator.ResultType(params Type[] operands) {
        return typeof(int);
    }
    #endregion
}

Imports System
Imports DevExpress.Data.Filtering
Imports DevExpress.Xpo.DB

' ...
Public Class MyGetMonthFunction
    Implements ICustomFunctionOperatorFormattable
    #Region "ICustomFunctionOperatorFormattable Members"
    ' The function's expression to be evaluated on the server.
    Private Function ICustomFunctionOperatorFormattable_Format(ByVal providerType As Type, _
    ParamArray ByVal operands() As String) As String _
    Implements ICustomFunctionOperatorFormattable.Format
        ' This example implements the function for Microsoft Access databases only.
        If providerType Is GetType(AccessConnectionProvider) Then
            Return String.Format("datepart(""m"", {0})", operands(0))
        End If
        Throw New NotSupportedException(String.Concat( _
        "This provider is not supported: ", providerType.Name))
    End Function

    Public Shared Function MyGetMonth(ByVal [date] As DateTime) As Integer
        Return [date].Month
    End Function
    #End Region

    #Region "ICustomFunctionOperator Members"
    ' Evaluates the function on the client.
    Private Function Evaluate(ParamArray ByVal operands() As Object) As Object _
    Implements ICustomFunctionOperator.Evaluate
        Return MyGetMonth(CDate(operands(0)))
    End Function

    Private ReadOnly Property Name() As String Implements ICustomFunctionOperator.Name
        Get
            Return NameOf(MyGetMonth)
        End Get
    End Property

    Private Function ResultType(ParamArray ByVal operands() As Type) As Type _
    Implements ICustomFunctionOperator.ResultType
        Return GetType(Integer)
    End Function
    #End Region
End Class

using System;
using DevExpress.Xpo;
using DevExpress.Xpo.DB;
using DevExpress.Xpo.Metadata;
using DevExpress.Data.Filtering;

static class Program {
    static void Main() {
        ConnectionProviderSql provider = 
            (ConnectionProviderSql)XpoDefault.GetConnectionProvider(
            AccessConnectionProvider.GetConnectionString(@"..\..\CustomFunction.mdb"),
            AutoCreateOption.DatabaseAndSchema);
        XPDictionary dict = new ReflectionDictionary();

        // New registration technique.
        CriteriaOperator.RegisterCustomFunction(new MyGetMonthFunction());

        // Outdated registration technique.
        // provider.RegisterCustomFunctionOperator(new MyGetMonthFunction());
        // dict.CustomFunctionOperators.Add(new MyGetMonthFunction());

        // Instantiating a DAL.
        XpoDefault.DataLayer = new SimpleDataLayer(dict, provider);
        XpoDefault.Session = null;
        // ...
    }
}

Imports System
Imports DevExpress.Xpo
Imports DevExpress.Xpo.DB
Imports DevExpress.Xpo.Metadata
Imports DevExpress.Data.Filtering

Friend NotInheritable Class Program
    Private Sub New()
    End Sub
    Shared Sub Main()
        Dim provider As ConnectionProviderSql = CType(XpoDefault.GetConnectionProvider( _
        AccessConnectionProvider.GetConnectionString("..\..\CustomFunction.mdb"), _
        AutoCreateOption.DatabaseAndSchema), ConnectionProviderSql)
        Dim dict As XPDictionary = New ReflectionDictionary()

        ' New registration technique.
        CriteriaOperator.RegisterCustomFunction(New MyGetMonthFunction())

        ' Outdated registration technique.
        ' provider.RegisterCustomFunctionOperator(new MyGetMonthFunction());
        ' dict.CustomFunctionOperators.Add(new MyGetMonthFunction());

        ' Instantiating a DAL.
        XpoDefault.DataLayer = New SimpleDataLayer(dict, provider)
        XpoDefault.Session = Nothing
        ' ...
    End Sub
End Class

static class Program {
    static void Main() {
        // ...
        using (Session session = new Session()) {
            // Using MyGetMonth to build criteria.
            CriteriaOperator criteria = CriteriaOperator.Parse("MyGetMonth(OrderDate) = 5");
            // Applying criteria to filter XPView contents.
            XPView view = new XPView(session, typeof(Order), "OrderDate", criteria);
            // Using MyGetMonth in an XPView column's expression.
            view.AddProperty("Month", "MyGetMonth(OrderDate)");

            foreach (ViewRecord prop in view) {
                Console.WriteLine(prop["OrderDate"]);
                Console.WriteLine("Month: " + prop["Month"]);
            }
        }
        Console.WriteLine("done\npress any key to exit ..");
        Console.ReadKey();
    }
}

Friend NotInheritable Class Program
    Private Sub New()
    End Sub
    Shared Sub Main()
        ' ...
        Using session As New Session()
            ' Using MyGetMonth to build criteria.
            Dim criteria As CriteriaOperator = CriteriaOperator.Parse("MyGetMonth(OrderDate) = 5")
            ' Applying criteria to filter XPView contents.
            Dim view As New XPView(session, GetType(Order), "OrderDate", criteria)
            ' Using MyGetMonth in an XPView column's expression.
            view.AddProperty("Month", "MyGetMonth(OrderDate)")

            For Each prop As ViewRecord In view
                Console.WriteLine(prop("OrderDate"))
                Console.WriteLine("Month: " & prop("Month"))
            Next prop
        End Using
        Console.WriteLine("done" & Constants.vbLf & "press any key to exit ..")
        Console.ReadKey()
    End Sub
End Class