Variable assignment within method parameter

Question

I've just found out (by discovering a bug) that you can do this:

string s = "3";
int i;
int.TryParse(s = "hello", out i); //returns false

Is there a legitimate use of using the returned value of an assignment?

_{(Obviously i++ is, but is this the same?)}

Answer 1

Generally I'd avoid using the return value of an assignment as it can all too easily lead to had to spot bugs. However, there is one excellent use for the feature as hopefully illustrated below, lazy initialization:

class SomeClass
{
    private string _value;

    public string Value { get { return _value ?? (_value = "hello"); } }
}

As of C# 6, this can be expressed using the => notation:

class SomeClass
{
    private string _value;

    public string Value => _value ?? (_value = "hello");
}

By using the ?? notation and the return value from the assignment, terse, yet readable, syntax can be used to only initialize the field and return it via a property when that property is called. In the above example, this isn't so useful, but within eg facades that need to be unit tested, only initializing those parts under test can greatly simplify the code.

Answer 2

This is legitimate.

s = "hello", is an expression which is evaluated / executed first, and the int.TryParse expression is executed after that.

Therefore, int.TryParse will use the content of 's' which is at that time "hello" and it's returning false.