Should a function have only one return statement?

I often have several statements at the start of a method to return for "easy" situations. For example, this:

public void DoStuff(Foo foo)
{
    if (foo != null)
    {
        ...
    }
}

... can be made more readable (IMHO) like this:

public void DoStuff(Foo foo)
{
    if (foo == null) return;

    ...
}

So yes, I think it's fine to have multiple "exit points" from a function/method.

Nobody has mentioned or quoted Code Complete so I'll do it.

17.1 return

Minimize the number of returns in each routine. It's harder to understand a routine if, reading it at the bottom, you're unaware of the possibility that it returned somewhere above.

Use a return when it enhances readability. In certain routines, once you know the answer, you want to return it to the calling routine immediately. If the routine is defined in such a way that it doesn't require any cleanup, not returning immediately means that you have to write more code.

I would say it would be incredibly unwise to decide arbitrarily against multiple exit points as I have found the technique to be useful in practice over and over again, in fact I have often refactored existing code to multiple exit points for clarity. We can compare the two approaches thus:-

string fooBar(string s, int? i) {
  string ret = "";
  if(!string.IsNullOrEmpty(s) && i != null) {
    var res = someFunction(s, i);

    bool passed = true;
    foreach(var r in res) {
      if(!r.Passed) {
        passed = false;
        break;
      }
    }

    if(passed) {
      // Rest of code...
    }
  }

  return ret;
}

Compare this to the code where multiple exit points are permitted:-

string fooBar(string s, int? i) {
  var ret = "";
  if(string.IsNullOrEmpty(s) || i == null) return null;

  var res = someFunction(s, i);

  foreach(var r in res) {
      if(!r.Passed) return null;
  }

  // Rest of code...

  return ret;
}

I think the latter is considerably clearer. As far as I can tell the criticism of multiple exit points is a rather archaic point of view these days.