Why Are My Extension Methods Failing with 'Extension methods must be defined in a non-generic static class'?

Extension Methods Must Be Defined in a Non-Generic Static Class

When attempting to define extension methods, you may encounter the error: "Extension methods must be defined in a non-generic static class." This indicates that your helper class does not meet the required criteria for creating extension methods in C#.

To rectify this issue, review the following guidelines:

1. Non-Generic Class:
   The class containing the extension method must be declared as non-generic, meaning it should not have type parameters within angle brackets (e.g., public class LinqHelper).
2. Static Class:
   Extension methods must be defined within static classes only. This means the class should be declared as public static class LinqHelper.
3. Non-Nested Class:
   Extension methods cannot be defined within nested classes. Thus, the LinqHelper class should be a standalone class without any enclosing classes.

In your provided code, the LinqHelper class was initially defined as a generic class, which violated requirement #1. Modifying it to public static class LinqHelper will address the error.

Extension Method Characteristics:

In addition to the class requirements, extension methods themselves must adhere to certain conventions:

1. Static Method:
   Extension methods must be declared as static within the non-generic static class.
2. This Keyword:
   The first parameter of the extension method uses the this keyword followed by the type to which the extension is being applied, followed by the name of the parameter. For example, public static IOrderedQueryable OrderBy(this IQueryable source, string property).

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