Compiler Error CS0233

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'identifier' does not have a predefined size, therefore sizeof can only be used in an unsafe context (consider using System.Runtime.InteropServices.Marshal.SizeOf)

The sizeof operator can only be used for types that are compile-time constants. If you are getting this error, make sure that the size of the identifier can be determined at compile time. If it cannot, then use SizeOf instead of sizeof.

Example

The following example generates CS0233:

// CS0233.cs
using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
public struct S
{
    public int a;
}

public class MyClass
{
    public static void Main()
    {
        S myS = new S();
        Console.WriteLine(sizeof(S));   // CS0233
        // Try the following line instead:
        // Console.WriteLine(Marshal.SizeOf(myS));
   }
}