C# where keyword

Question

In the following piece of code (C# 2.0):

public abstract class ObjectMapperBase< T > where T : new()
{
    internal abstract bool UpdateObject( T plainObjectOrginal,
                                         T plainObjectNew,
                                         WebMethod fwm,
                                         IDbTransaction transaction );
}

Inheritor example:

public abstract class OracleObjectMapperBase< T > : ObjectMapperBase< T > where T : new()
{
    internal override bool UpdateObject( T plainObjectOrginal,
                                         T plainObjectNew,
                                         WebMethod fwm,
                                         IDbTransaction transaction )
    {
        // Fancy Reflection code.
    }
}

What does the where keyword do?

score 15 · Accepted Answer · edited Jan 25 '19 at 13:12

15

it is a constraint for generics

MSDN

so the new() constraint says it must have a public parameterless constructor

edited Jan 25 '19 at 13:12

specializt

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answered May 05 '10 at 14:25

Pharabus

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Jeff Sternal · Answer 2 · 2010-05-05T14:42:16.250

9

It specifies a constraint on the generic type parameter T.

The new() constraint specifies that T must have a public default constructor.

You can also stipulate that the type must be a class (or conversely, a struct), that it must implement a given interface, or that it must derive from a particular class.

edited May 05 '10 at 14:42

answered May 05 '10 at 14:25

Jeff Sternal

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I think in this case, it specifies that `T` must have a public no-arg constructor. – FrustratedWithFormsDesigner May 05 '10 at 14:27
1

@Frustrated - indeed: I've expanded on my original ridiculously terse answer. ;) – Jeff Sternal May 05 '10 at 14:30

score 5 · Answer 3 · answered May 05 '10 at 14:26

The where clause is used to specify constraints on the types that can be used as arguments for a type parameter defined in a generic declaration. For example, you can declare a generic class, MyGenericClass, such that the type parameter T implements the IComparable interface:

public class MyGenericClass<T> where T:IComparable { }

In this particular case it says that T must implement a default constructor.

score 3 · Answer 4 · answered May 05 '10 at 14:26

3

This is a generic type constraint. It means that the generic type T must implement a zero parameter constructor.

answered May 05 '10 at 14:26

Oded

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score 3 · Answer 5 · answered May 05 '10 at 14:27

The Where keyword is basically a constraint on the objects the class can work on/with.

taken from MSDN "The new() Constraint lets the compiler know that any type argument supplied must have an accessible parameterless constructor"

http://msdn.microsoft.com/en-us/library/6b0scde8(VS.80).aspx

score 1 · Answer 6 · answered May 05 '10 at 14:25

1

It means the T has to have a public default constructor.

answered May 05 '10 at 14:25

Codism

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C# where keyword

6 Answers6