What is the purpose of new() while declaration of a generic class?

What is the purpose of new() while declaration of BaseEntityCollection class?

If I'm going to remove it, I got an error with the following message "T must be a non-abstract type with a public parameterless constructor in order to use it as parameter ..."

public abstract 开发者_StackOverflow中文版partial class BaseEntityCollection<T> : 
       List<T> where T : BaseEntity, new()

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It means that whatever class you specify for T, it has a default (no parameters) constructor.

Therefore, in the generic class, you can do new T() and it will create a new object of type T.

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Writing new() forces the parameter to have a default constructor.

Without it, you can't write new T().
Your error happens when you try to pass a non-new() type as a new()'d parameter.

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Also, do not inherit List<T>.
Instead, you should inherit Collection<T>, which is designed for inheritance.

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The type T has to have a parameterless constructor. This enables you to create new instances by doing var t = new T() which would be impossible otherwise.

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It is the notation for the generic constraint: Must have (public) parameterless constructor.

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This means that your generic type has to have parameterless constructor.

BaseEntityCollection<T> : List<T>

I am not sure what are you doing here, but I think it is against Liskov's rule. Check your hierarchy.

Constraints on Type Parameters

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That is one of the possible "generic type constraints" you can associate with your generic type. Using the constraint "new()" will only allow you to use a generic type if it has a parameterless constructor. This can be useful for things like serialization, or factory-type methods, where you need to create an object of type T.

Here are some other generic type constraints: http://msdn.microsoft.com/en-us/library/d5x73970(v=vs.80).aspx

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It's a generic constraint. In this case, it's a the new Constraint.