Generic Constraint for Non Nullable types

Question

I have the following class:

public class KeyDTO<T>
{
     public T Id { get; set; }
}

So far so good, but I want the type parameter T to be a non-nullable type. I've read somewhere that this may be feasible:

public class KeyDTO<T> where T : IComparable, IComparable<T>
{
     public T Id { get; set; }
}

But, If i change public T Id to public T? Id, I get a compilation error telling me that T must be non-nullable.

How can I specify that a generic type parameter must be non-nullable?

Edit

I want to accomplish this because I want to annotate my Id property with the [Required] attribute as follows:

public class KeyDTO<T> {
    [Required]
    public T Id { get; set; }
}

What [Required] does is validate the model so T cannot be null.

However, if I have KeyDTO<int>, Id will be initialized to 0, bypassing my [Required] attribute

Solution 1

Applying where T : struct applies a generic constraint that T be a non-nullable value type. Since there are no non-nullable reference types, this has the exact same semantics as simply "all non-nullable types". Nullable value types (i.e. Nullable<T>) do not satisfy the struct generic constraint.

Solution 2

From C# 8.0 you can now use the where T : notnull generic constraint to specificy T is a non-nullable type.

Comments

[Christos]

Good explanation (+1). But I have a question: din't Nullable<T> requires where T : struct. Isn't a problem. Thank you in advance.

[Servy]

I have no idea what you're trying to ask.

[Christos]

Since Nullable is a value type, would inherit from struct. Correct? If so, how instructing where T : struct exclude the non-nullable types? Does this make sense? I am sorry but I can't get it.

[Servy]

@Christos It doesn't apply to that constraint because that's what the C# language specs say should happen. Note there is no struct type. It is not something that is inherited from. It is a concept; some types satisfy it, and some don't. It's that simple.

[John Melville]

Please notice the date on this answer, it was true when it was written. In 2019 c# 8.0 adds non-nullable reference types to C#.

[Guvante]

"Generic declarations that include the notnull constraint can be used in a nullable oblivious context, but compiler does not enforce the constraint." So if you aren't compiling with nullable enable it won't do anything.

[Liero]

nullable reference types are finally enabled in .NET 6 project templates by default

[Noman_1]

I'm using .Net Core 3.1 with "Nullable: enable" on project build config. This worked for me