"In Parameter Specification at Call Site for Explicit Pass by Reference"

// Second case: Forcing the passing of an 'in' parameter to be a reference to live variable 

static void PrintFirstElement(in Matrix4x4 m) => Console.WriteLine(m.M11);

static void Test() {
	// Matrix4x4.Identity is a static property that returns a new Matrix4x4

	PrintFirstElement(Matrix4x4.Identity); // Compiles, because the compiler creates a temporary variable on the stack that is what is referred to
    PrintFirstElement(in Matrix4x4.Identity); // Fails, because we're creating a reference to something that only exists as a temporary variable

Code snippet taken from "Two Decades of C#: A Reference - C# 7".