Call one constructor from the body of another in C#

You can’t.

You’ll have to find a way to chain the constructors, as in:

public foo (int x, int y) { }
public foo (string s) : this(XFromString(s), YFromString(s)) { ... }

or move your construction code into a common setup method, like this:

public foo (int x, int y) { Setup(x, y); }
public foo (string s)
{
   // do stuff
   int x = XFromString(s);
   int y = YFromString(s);
   Setup(x, y);
}

public void Setup(int x, int y) { ... }