Is there a standard "never returns" attribute for C# functions?

Why not just change it to

int foo(int x, y)
{
    if (y == 0)
        throwException("Doh!");
    return x/y;
}

This gives the same runtime results, and the compiler won't complain.

No. I suggest you change the signature of your first function to return the exception rather than throw it, and leave the throw statement in your second function. That'll keep the compiler happy, and smells less bad as well.

Edit: there is now a DoesNotReturn attribute that provides an alternative.

Bernhof's answer is correct. However, if you are trying to encapsulate a large chunk of logic when instantiating your exception, then all you need to do is change your code from this:

void throwException(string msg) {
    throw new MyException(msg);
}

to this:

Exception makeException(string msg) {
    return new MyException(msg);
}

Then your calling code will look like this:

int foo(int x, y) {
    if (y == 0) {
        throw makeException("Doh!");
    }
    return x / y;
}

All other things being equal, prefer functional code to procedural code. It's easier to re-use and unit-test.

EDIT:

In light of Fred's sample code, this is what I would do. It's not a code contract, but it's still functional.

private int getVarID(string s_varID) {
    int varID;
    if(s_varID == "ILT") {
        return 123;
    } else if(s_varID == "TL") {
        return 456;
    } else if(s_varID == "FT") {
        return 789;
    } else if(int.TryParse(s_varID, out varID)) {
        return varID;
    } else {
        throw makeParseError("varID must be an integer or 'ILT', 'TL' or 'FT'.");
    }
}

The [DoesNotReturn] attribute in System.Diagnostics.CodeAnalysis should be what you want.

Reference: https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.doesnotreturnattribute?view=netcore-3.1

Don't hand the exception creation off to another function (i.e. just throw it directly) and the compiler won't complain. Handing off to a "helper" type function for exception throwing is a waste of time unless the function is actually adding value to the exception process.

Your function

void throwException(string msg)
{
    throw new MyException(msg);
}

add zero value to the code, hence your question is moot. If, on the other hand you want to throw an error with the same message throughout the class and minimise code duplication this is what you should do.

The normal practice would be to extend MyException for this particular case and throw that:

public class HomerSimpsonException : MyException
{
   public HomerSimpsonException() : base ("DOH!!!"){
   }
}
int foo(int x, y)
{
    if (y == 0)
        throw new HomerSimpsonException();
    else
        return x/y;
}

Even then, that's not complete enough as per Microsoft rule for extending exceptions, there are minimum 4 constructors that you should implement - http://msdn.microsoft.com/en-us/library/ms182151%28VS.80%29.aspx, namely:

  public NewException(){}
  public NewException(string){}
  public NewException(string, Exception){}
  protected or private NewException(SerializationInfo, StreamingContext){}

Bernhof already gave you a way to avoid the compiler complain. However, also be aware your stack trace will be off (and some logger libraries won't handle util-classed-i-throw-exceptions-for-your-app methods) which makes debugging your application harder.

Remove the 'else' keyword, it's redundant anyway, and it will work ;)