这两种定义有什么区别？

Question

string a = "var";
string b = new string("var");

I also have an extra question, How do I access the string type variable in the 1st definition without allocating space in the heap?

Answer 1

.NET Framework 4.8:

The first one compiles; the second one doesn't, producing a CS1503 error:

Argument 1: cannot convert from 'string' to 'char*'

That is because the String class does not have a constructor which accepts a string parameter.

.NET Core 2.1 and later

The first one assigns the literal to a string variable. The second one implicitly casts the string to a ReadOnlySpan<char> , then calls the String constructor which accepts a ReadOnlySpan<char> instance, which copies the characters from the original string to a new string, doubling the memory usage.

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As to your second question, since the String is a reference type, you cannot allocate an instance without allocating space in the heap.