Writing Nullable value with BinaryWriter [duplicate]

Question

This question already has answers here:

C# nullable string error (5 answers)

Closed 8 years ago.

I want to write a nullable string to file using BinaryWriter and this code:

  BinaryWriter writer = new BinaryWriter(st);
  String? s;
  if(s!=null){
        writer.Write(s);
    }

but this error occurs:

The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

How can I solve this problem?

Answer 1

Strings can always be null, you don't need to try and make it nullable as they're not value types. Just remove the ? That makes it nullable to fix it.

Answer 2

Strings are already nullable in C#.NET, they're a reference type with syntactic sugar around them to make them easier to use. You can't make a nullable double nullable, the universe would collapse.

Answer 3

According to MSDN documentation, we can see that Nullable type can be used only with structs:

[SerializableAttribute]
public struct Nullable<T>
where T : struct

p.s. see also Constraints on Type Parameters.

Answer 4

String is a reference type. It can not be nullable.
public struct Nullable<T> where T : struct, new()