Extension methods must be defined in a non-generic static class

Question

I'm getting the error:

Extension methods must be defined in a non-generic static class

On the line:

public class LinqHelper

Here is the helper class, based on Mark Gavells code. I'm really confused as to what this error means as I am sure it was working fine when I left it on Friday!

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Linq.Expressions;
using System.Reflection;

/// <summary>
/// Helper methods for link
/// </summary>
public class LinqHelper
{
    public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "OrderBy");
    }
    public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "OrderByDescending");
    }
    public static IOrderedQueryable<T> ThenBy<T>(this IOrderedQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "ThenBy");
    }
    public static IOrderedQueryable<T> ThenByDescending<T>(this IOrderedQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "ThenByDescending");
    }
    static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, string methodName)
    {
        string[] props = property.Split('.');
        Type type = typeof(T);
        ParameterExpression arg = Expression.Parameter(type, "x");
        Expression expr = arg;
        foreach (string prop in props)
        {
            // use reflection (not ComponentModel) to mirror LINQ
            PropertyInfo pi = type.GetProperty(prop);
            expr = Expression.Property(expr, pi);
            type = pi.PropertyType;
        }
        Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
        LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);

        object result = typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { source, lambda });
        return (IOrderedQueryable<T>)result;
    }
}

Answer 1

change

public class LinqHelper

to

public static class LinqHelper

Following points need to be considered when creating an extension method:

1. The class which defines an extension method must be non-generic, static and non-nested
2. Every extension method must be a static method
3. The first parameter of the extension method should use the this keyword.

Answer 2

if you do not intend to have static functions just get rid of the "this" keyword in the arguments.

Answer 3

Add keyword static to class declaration:

// this is a non-generic static class
public static class LinqHelper
{
}

Answer 4

A work-around for people who are experiencing a bug like Nathan:

The on-the-fly compiler seems to have a problem with this Extension Method error... adding static didn't help me either.

I'd like to know what causes the bug?

But the work-around is to write a new Extension class (not nested) even in same file and re-build.

Figured that this thread is getting enough views that it's worth passing on (the limited) solution I found. Most people probably tried adding 'static' before google-ing for a solution! and I didn't see this work-around fix anywhere else.

Answer 5

Try changing

public class LinqHelper

to

 public static class LinqHelper

Answer 6

I was scratching my head with this compiler error. My class was not an extension method, was working perfectly since months and needed to stay non-static. I had included a new method inside the class:

private static string TrimNL(this string Value)
{...}

I had copied the method from a sample and didn't notice the "this" modifier in the method signature, which is used in extension methods. Removing it solved the issue.

Answer 7

Change it to

public static class LinqHelper

Answer 8

I ran into this when converting a project to use dependency injection. If a method declaration contains the “this” keyword, VS will give the warning. In my case, I was able to remove “this” from all of the method declarations. If using “this” is required, you will have to make it static.

 public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)

changed to

     public static IOrderedQueryable<T> OrderBy<T>(IQueryable<T> source, string property)

You might need to code around not using the “this” keyword for the method declaration if you want to avoid using a static class with static methods. If “this” is only required for some methods, those methods can be moved to a separate public static class.

Answer 9

Extension method should be inside a static class. So please add your extension method inside a static class.

so for example it should be like this

public static class myclass
    {
        public static Byte[] ToByteArray(this Stream stream)
        {
            Int32 length = stream.Length > Int32.MaxValue ? Int32.MaxValue : Convert.ToInt32(stream.Length);
            Byte[] buffer = new Byte[length];
            stream.Read(buffer, 0, length);
            return buffer;
        }

    }

Answer 10

Try changing it to static class and back. That might resolve visual studio complaining when it's a false positive.

Answer 11

I encountered a similar issue, I created a 'foo' folder and created a "class" inside foo, then I get the aforementioned error. One fix is to add "static" as earlier mentioned to the class which will be "public static class LinqHelper".

My assumption is that when you create a class inside the foo folder it regards it as an extension class, hence the following inter alia rule apply to it:

1) Every extension method must be a static method

WORKAROUND If you don't want static. My workaround was to create a class directly under the namespace and then drag it to the "foo" folder.