How to fix: ‘Point’ does not have a predefined size, therefore sizeof can only be used in an unsafe context

The error you’re encountering is because in C#, sizeof can only be used with types that have a predefined size known at compile-time, and the Point structure is not one of those types unless you’re in an unsafe context.

There are two ways you can resolve this.

Use unsafe code

This would allow the use of the sizeof operator with types of any size. To do this, you’ll need to mark your method with the unsafe keyword, and you’ll also need to enable unsafe code in your project’s build settings.

Basically, your method signature changes to this:

public static unsafe void YourMethod()
{
    // ... your unsafe code
    // IntPtr sizeOfPoint = (IntPtr)sizeof(Point);
}
Code language: C# (cs)

And for allowing unsafe code, you go to project properties, and into the Build tab, and check the “Allow unsafe code” option. Once you’ve done this, the compilation error should be gone.

Use Marshal.SizeOf

Marshal.SizeOf is safe and doesn’t require unsafe context. The SizeOf method returns the unmanaged size of an object in bytes.

All you need to do is replace sizeof(Point) with Marshal.SizeOf(typeof(Point)). Like so:

IntPtr sizeOfPoint = (IntPtr)Marshal.SizeOf(typeof(Point));
Code language: C# (cs)

Marshal.SizeOf is part of the System.Runtime.InteropServices namespace, so ensure you have the using directive for it at the top of your file:

using System.Runtime.InteropServices;
Code language: C# (cs)

One thing to note is that Marshal.SizeOf does come with a very slight performance penalty compared to the unsafe sizeOf. That is something you might want to take into consideration when choosing the solution that best suits your needs.