C# 9 Unconstrained Type Parameter Annotations

C# 9.0 allows nullable annotations for type parameters that are not constrained to value types or reference types such as, T?.

• In C# 8.0, the nullable (?) annotations could only be applied to type parameters that were explicitly constrained to value types or reference types.
• In C# 9.0, it can be applied to any type of parameter, regardless of constraints.
• Unless a type parameter is explicitly constrained to value types, annotations can only be applied within a #nullable enable context.

If a type parameter T is substituted with a reference type, then T? represents a nullable instance of that reference type. If T is substituted with a value type, then T? represents an instance of T.

var s1 = new string[2] { "str1", "str2" };  // string? s1
var s2 = new string?[2] { "str3", "str4" }; // string? s2

var i1 = new int[2] { 1, 2 };  // int i1
var i2 = new int?[2] { 3, 4 }; // int? i2

If T is substituted with an annotated type U?, then T? represents the annotated type U? rather than U??.

var u1 = new U[0].FirstOrDefault();  // U? u1
var u2 = new U?[0].FirstOrDefault(); // U? u2

If T is substituted with a type U, then T? represents U?, even within a #nullable disable context.

Default Constraint

For compatibility with existing code were overridden and explicitly implemented generic methods could not include explicit constraint clauses, T? in an overridden or explicitly implemented method is treated as Nullable<T> where T is a value type.

To allow annotations for type parameters constrained to reference types, C#8 allowed explicit where T : class and where T : struct constraints on the overridden or explicitly implemented method.

class A1
{
    public virtual void F1<T>(T? t) where T : struct { }
    public virtual void F1<T>(T? t) where T : class { }
}

class B1 : A1
{
    public override void F1<T>(T? t) /*where T : struct*/ { }
    public override void F1<T>(T? t) where T : class { }
}

To allow annotations for type parameters that are not constrained to reference types or value types, C#9 allows a new where T : default constraint.

class A2
{
    public virtual void F2<T>(T? t) where T : struct { }
    public virtual void F2<T>(T? t) { }
}

class B2 : A2
{
    public override void F2<T>(T? t) /*where T : struct*/ { }
    public override void F2<T>(T? t) where T : default { }
}

It is an error to use a default constraint in the following scenarios.

• Other than on a method override or explicit implementation.
• When the corresponding type parameter in the overridden or interface method is constrained to a reference type or value type.