CODEWITHSUNDEEP
c#generics
JensOlsen112
JensOlsen112

Reputation: 1299

Issue when creating generic extension method

Right now I'm trying to create a generic method for including foreign keys in my repository.

What I currently got is this:

public static class ExtensionMethods
{
    private static IQueryable<T> IncludeProperties<T>(this DbSet<T> set, params Expression<Func<T, object>>[] includeProperties)
    {
        IQueryable<T> queryable = set;
        foreach (var includeProperty in includeProperties)
        {
            queryable = queryable.Include(includeProperty);
        }

        return queryable;
    }
}

However when compiling I get the error:

The type 'T' must be a reference type in order to use it as parameter 'TEntity' in the generic type or method 'System.Data.Entity.DbSet'

What could be the issue here?

Upvotes: 2

Views: 786

Answers (2)

Servy
Servy

Reputation: 203802

As the error message states, the generic argument to DbSet must be a reference type. Your generic argument could be anything, including non-reference types. You need to constraint it to just reference types.

Upvotes: 0

Mike Strobel
Mike Strobel

Reputation: 25623

Append where T : class to the end of your method signature:

private static IQueryable<T> IncludeProperties<T>(
    this DbSet<T> set,
    params Expression<Func<T, object>>[] includeProperties)
    where T : class // <== add this constraint.
{
    ...
}

DbSet<TEntity> has this constraint, so in order for your T type argument to be compatible with TEntity, it must have the same constraint.

Upvotes: 7

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