meet it

A tells C# what your type can do.

Inside a generic method, C# knows nothing about T, so it only lets you use what every object has, like Equals. To find the biggest item you must compare, and plain T has no CompareTo. Writing item.CompareTo(best) stops with CS1061.

A constraint fixes this. Write where T : IComparable<T> after the brackets, and only types that can compare themselves are allowed in. string, int and double all implement IComparable, so inside the method CompareTo is ready to use.

a.CompareTo(b) answers with a number: below zero when a comes first, zero when they match, above zero when a comes after. Check the sign, never the exact value.

using System;
using System.Collections.Generic;

Console.WriteLine(Biggest(new List<int> { 4, 19, 7 }));
Console.WriteLine(Biggest(new List<string> { "kiwi", "apple", "plum" }));
Console.WriteLine(Biggest(new List<double> { 2.5, 0.75 }));

T Biggest<T>(List<T> items) where T : IComparable<T>
{
    T best = items[0];
    foreach (T item in items)
    {
        if (item.CompareTo(best) > 0)
        {
            best = item;
        }
    }
    return best;
}

Every built-in number type, including decimal for money (written 3.40m), implements IComparable<T>.

guess first

Keep values inside a range. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

using System;

Console.WriteLine(Clamp(15, 0, 10));
Console.WriteLine(Clamp(-3, 0, 10));
Console.WriteLine(Clamp("m", "a", "f"));

T Clamp<T>(T value, T low, T high) where T : IComparable<T>
{
    if (value.CompareTo(low) < 0)
    {
        return low;
    }
    if (value.CompareTo(high) > 0)
    {
        return high;
    }
    return value;
}

your turn

Your turn. Find the smallest of anything.

Add a constraint in the gap on line 9 so CompareTo works. Then fill line 5 with three town names of your own, in quotes.

Your code

output

press run to see what happens

fix it

The bargain finder picks the dearest price. Can you fix it?

Broken code to fix

output

press run to see what happens

The code runs, but it shows 3.40. Look at the test inside the loop.
CompareTo is above zero when item comes after best, so this keeps the biggest.
Change > 0 to < 0 on line 12, then run.

nailed it

your generic code can compare now. :D

A constraint says what T must be able to do.
where T : IComparable<T> unlocks CompareTo.
Check the sign of CompareTo, never the exact number.

new words in your collection

constraint

This lesson isn’t saved. A free account remembers every lesson you finish.

A constraint tells C# what your type can do.

Keep values inside a range. What will this show?

Your turn. Find the smallest of anything.

The bargain finder picks the dearest price. Can you fix it?

your generic code can compare now. :D

A tells C# what your type can do.