C# test if variable assigned

Question

I'm trying to code a Linq MinBy extension method

public static class Extensions
{
    public static T MinBy<T>(this IEnumerable<T> source, Func<T,int> selector)
    {
        T min;
        int? minKey = null;
        foreach (var x in source)
        {
            var key = selector(x);
            if (minKey == null || key < minKey)
            {
                minKey = key;
                min = x;
            }
        }
        if (minKey == null)
        {
            throw new ArgumentException("source should not be empty");
        }
        return min;
    }
}

I think my logic is correct and readable. But I get a build error

Use of unassigned local variable 'min'

What can I do about this? Can I test if the variable is assigned?

---

Clarification: A MinBy function can answer the following question. Which of the numbers [-5, -2, 3] has the smallest square?

> new List<int>{-5,-2,3}.MinBy(x => x*x)
-2

.NET's Min function answers a different question (which is the smallest of the squares)

> new List<int>{-5,-2,3}.Min(x => x*x)
4

Answer 1

You need a default value for min like this:

T min = default(T);

You can read more about default() on MSDN:

Given a variable t of a parameterized type T, the statement t = null is only valid if T is a reference type and t = 0 will only work for numeric value types but not for structs. The solution is to use the default keyword, which will return null for reference types and zero for numeric value types. For structs, it will return each member of the struct initialized to zero or null depending on whether they are value or reference types. For nullable value types, default returns a System.Nullable, which is initialized like any struct.

Answer 2

Add a default value for min:

T min = default(T);

The reason it's complaining is that the compiler cannot verify that min will have been assigned a value before it's used in the return min; line. An unassigned local variable cannot be referenced, and so the compiler generates an error.