List<> of struct with property. Cannot change value of property. why?

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This returns the following error:
"Cannot modify the return value of
'System.Collections.Generic.List<MyStruct>.this[int]' because it is
not a variable"
and I have no idea why! Do lists return copies of their elements?
Why can't I change the element itself?

class Program
{
private struct MyStruct
{
private int myVar;
public int MyProperty
{
get { return myVar; }
set { myVar = value; }
}
}

private static List<MyStruct> list = new List<MyStruct>();

private static void Main(string[] args)
{
MyStruct x = new MyStruct();
x.MyProperty = 45;
list.Add(x);
list[0].MyProperty = 45; // <----------- ERROR HERE
}
}

Zytan

.

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