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.