"where" keyword in class declaration in c sharp

Question

Hi, Could anyone help me with the line where TEntity : class, IEntity, new() in the following class declaration.

public abstract class BaseEntityManager<TEntity>
        where TEntity : class, IEntity, new()

Thanks for your help in advance.

Answer 1

where TEntity : ... applies constraints to the generic parameter TEntity. In this case, the constraints are:

class: The argument to TEntity must be a reference type
IEntity: The argument must be or implement the IEntity interface
new(): The argument must have a public parameterless constructor

From http://msdn.microsoft.com/en-us/library/d5x73970.aspx

Answer 2

Where is a generic type constraint. That lines reads that the type TEntity must be a reference type as opposed to a value type, must implement the interface IEntity and it must have a constructor that takes no parameters.

http://msdn.microsoft.com/en-us/library/bb384067.aspx

Answer 3

The where keyword after the class declaration restrict what type the generic TEntity could be. In this case TEntity must be a class (meaning it can't be a value type like int or DateTime), and it must implement the interface IEntity. The new() constraint indicates that methods inside this class have the ability to call the default constructor of the generic class represented by TEntity (e.g. new TEntity())

Answer 4

What's the question ?

Let me take at shot at what i think the question is. The constraint ensures that you can only subclass BaseEntityManager with a generic parameter which is a reference type implementing IEntity and containing a parameterless constructor.

E.X.

public class Product : IEntity {
  public Product() {}
}

public class Wrong {
  public Wrong() {}
}

public class WrongAgain : IEntity {
   private Wrong() {}
}


// compiles
public ProductManager : BaseEntityManager<Product> {}


// Error - not implementing IEntity
public WrongManager : BaseEntityManager<Wrong> {}


/ Error - no public parameterless constructor
public WrongAgainManager : BaseEntityManager<WrongAgain> {}

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