当函数可以返回MemoryStream时，为什么我不能声明MemoryStream为空（MemoryStream？）？

Question

I have a function that returns MemoryStream? . So null if there was an error that occurred. Then I found that I cannot declare a variable MemoryStream?

public MemoryStream? GetResponseStream() { }
MemoryStream? stream = GetResponseStream();

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

Answer 1

MemoryStream is a reference type (declared with the class keyword) and therefore already is nullable by itself. Only value types (declared with the struct keyword) are non-nullable and can be made nullable with ? .

So your method should look like this:

public MemoryStream GetResponseStream() { ... }

and your method call like this:

MemoryStream stream = GetResponseStream();
if (stream == null) { ... }

BTW: You might want to consider using exceptions to signal that an error occurred in GetResponseStream rather than returning null .

Answer 2

MemoryStream is a reference type so can be null. Only value types can be made into Nullable<T> because they are not allowed to be assigned a null value otherwise.

Answer 3

Only value types can be nullable, not reference types. A MemoryStream can already be null, so it doesn't make sense to make it nullable

Answer 4

There is no need for ? as reference types can be null .

public MemoryStream GetResponseStream()
{
    return(null);
}

Answer 5

The nullable modifier (?) is for use only with value types. A stream is an object type, which can always be set to null ( it is. by its nature, already 'nullable'). So there is no need to o what you are trying to do.