Why do I get “error: … must be a reference type” in my C# generic method?

Question

In various database tables I have both a property and a value column. I\'m using Linq to SQL to access the database.

I\'m writing a method which returns a dictionary

Answer 1

public class TEntityRepository<TEntity> : EFRepository<TEntity> , ITEntityRepository<TEntity>
    where TEntity : class, new()
{
}

Answer 2

This happens because of how Table<T> is declared:

public sealed class Table<TEntity> : IQueryable<TEntity>, 
    IQueryProvider, IEnumerable<TEntity>, ITable, IQueryable, IEnumerable, 
    IListSource
where TEntity : class  // <-- T must be a reference type!

The compiler is complaining because your method has no constraints on T, which means that you could accept a T which doesn't conform to the specification of Table<T>.

Thus, your method needs to be at least as strict about what it accepts. Try this instead:

private static Dictionary<string, string> GetProperties<T>(Table<T> table) where T : class

Answer 3

Just add the constraint where T : class to your method declaration.

This is required because Table<TEntity> has a where TEntity : class constraint. Otherwise your generic method could be called with a struct type parameter, which would require the CLR to instantiate Table<TEntity> with that struct type parameter, which would violate the constraint on Table<TEntity>.