C# Language Specification

Fixed size buffers in expressions

Member lookup (§7.3) of a fixed size buffer member proceeds exactly like member lookup of a field.

A fixed size buffer can be referenced in an expression using a simple-name (§7.5.2) or a member-access (§7.5.4).

When a fixed size buffer member is referenced as a simple name, the effect is the same as a member access of the form this.I, where I is the fixed size buffer member.

In a member access of the form E.I, if E is of a struct type and a member lookup of I in that struct type identifies a fixed size member, then E.I is evaluated an classified as follows:

• 
  If the expression E.I does not occur in an unsafe context, a compile-time error occurs.
  
• 
  If E is classified as a value, a compile-time error occurs.
  
• 
  Otherwise, if E is a moveable variable (§18.3) and the expression E.I is not a fixed-pointer-initializer (§18.6), a compile-time error occurs.
  
• 
  Otherwise, E references a fixed variable and the result of the expression is a pointer to the first element of the fixed size buffer member I in E. The result is of type S*, where S is the element type of I, and is classified as a value.
  

The subsequent elements of the fixed size buffer can be accessed using pointer operations from the first element. Unlike access to arrays, access to the elements of a fixed size buffer is an unsafe operation and is not range checked.

The following example declares and uses a struct with a fixed size buffer member.

unsafe struct Font
{
public int size;
public fixed char name[32];
}

class Test

{
unsafe static void PutString(string s, char* buffer, int bufSize) {
int len = s.Length;
if (len > bufSize) len = bufSize;
for (int i = 0; i < len; i++) buffer[i] = s[i];
for (int i = len; i < bufSize; i++) buffer[i] = (char)0;
}

unsafe static void Main()

{
Font f;
f.size = 10;
PutString("Times New Roman", f.name, 32);
}
}

3.