Gary's Blog

In a previous blog post I showed you how to use custom functions everywhere you could use a CriteriaOperator. In this post, I’m going to show you how to use custom functions in LINQ to XPO.

There are two interfaces that support this functionality, they are:

1. ICustomFunctionOperatorQueryable – which provides information about the method that will be associated with the custom function.
2. ICustomCriteriaOperatorQueryable – which provides information about the method that will be associated with custom criteria, and implements the behaviour to convert a LINQ expression to a CriteriaOperator.

Right, let’s write some code! To start with let’s open the project, from the previous post, in Visual Studio 2008 or greater, set target framework to 3.5 or greater and add a reference to the DevExpress.Xpo.v11.1.Linq assembly.

Next, in the MyConcatFunction class, add the implementation of ICustomFunctionOperatorQueryable

using DevExpress.Xpo.Helpers;
//...
public class MyConcatFunction : ICustomFunctionOperatorBrowsable, 
    ICustomFunctionOperatorFormattable, 
    ICustomFunctionOperatorQueryable 
{
    //The method name must be the same as the function name,
    //but it doesn't have to be in the same class
    public static string MyConcat(string string0, string string1, string string2, string string3) {
        return string.Concat(string0, string1, string2, string3);
    }
    #region ICustomFunctionOperatorQueryable Members
     
    public System.Reflection.MethodInfo GetMethodInfo() {
        return typeof(MyConcatFunction).GetMethod("MyConcat");
    }
    #endregion
}

Finally we will drop another GridControl (imaginatively called GridControl2 ) onto the form and add the following code to the form constructor:

var linqResult = from p in new XPQuery<Person>(Session.DefaultSession)
                 select new
                 {
                     p.FirstName,
                     p.LastName,
                     NameLinqToXpo =
                         MyConcatFunction.MyConcat(p.FirstName,
                         " ", 
                         p.LastName, 
                         " (Linq To Xpo)")
                 };
gridControl2.DataSource = linqResult.ToList();

Having done that, running the application will give us this result:

That’s all for this short post, until next time, happy coding!