C# Tip: List Pattern to match an collection against a sequence of patterns

By using list patterns on an array or a list you can check whether a it contains the values you expect in a specific position.

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With C# 11 we have an interesting new feature: list patterns.

You can, in fact, use the is operator to check if an array has the exact form that you expect.

Take this method as an example.

Introducing List Patterns

string YeahOrError(int[] s)
{
    if (s is [1, 2, 3]) return "YEAH";
    return "error!";
}

As you can imagine, the previous method returns YEAH if the input array is exactly [1, 2, 3]. You can, in fact, try it by running some tests:

[Test]
public void PatternMatchingWorks()
{
    Assert.That(YeahOrError(new int[] { 1, 2, 3 }), Is.EqualTo("YEAH"));
    Assert.That(YeahOrError(new int[] { 1, 2, 3, 4 }), Is.EqualTo("error!"));
    Assert.That(YeahOrError(new int[] { 2, 3, 1}), Is.EqualTo("error!"));
}

As you can see, if the order is different, the check does not pass.

List Patterns with Discard

We can also use discard values to check whether a list contains a specific item in a specified position, ignoring all the other values:

string YeahOrErrorWithDiscard(int[] s)
{
    if (s is [_, 2, _]) return "YEAH";
    return "error!";
}

So, to be valid, the array must have exactly 3 elements, and the second one must be a “2”.

[Test]
public void PatternMatchingWorksWithDiscard()
{
    Assert.That(YeahOrErrorWithDiscard(new int[] { 1, 2, 3 }), Is.EqualTo("YEAH"));
    Assert.That(YeahOrErrorWithDiscard(new int[] { 9, 2, 6 }), Is.EqualTo("YEAH"));
    Assert.That(YeahOrErrorWithDiscard(new int[] { 1, 6, 2, 3 }), Is.EqualTo("error!"));
    Assert.That(YeahOrErrorWithDiscard(new int[] { 6, 3, 8, 4 }), Is.EqualTo("error!"));
}

List Patterns with variable assignment

You can also assign one or more of such values to a variable, and discard all the others:

string SelfOrMessageWithVar(int[] s)
{
    if (s is [_, 2, int third]) return "YEAH_" + third;
    return "error!";
}

The previous condition, s is [_, 2, int third], returns true only if the array has 3 elements, and the second one is “2”. Then, it stores the third element in a new variable, int third, and uses it to build the returned string.

[Test]
public void can_use_list_patterns_with_var()
{
    Assert.That(SelfOrMessageWithVar(new int[] { 1, 2, 3 }), Is.EqualTo("YEAH_3"));
    Assert.That(SelfOrMessageWithVar(new int[] { 1, 6, 2, 3 }), Is.EqualTo("error!"));
    Assert.That(SelfOrMessageWithVar(new int[] { 6, 3, 8, 4 }), Is.EqualTo("error!"));
}

List Patterns with item constraints

Finally, you can also specify further constraints on each value in the condition, using operators such as or, >, >=, and so on.

string SelfOrMessageWithCondition(int[] s)
{
    if (s is [0 or 1, > 2, int third]) return "YEAH_" + third;
    return "error!";
}

You can easily guess the meaning of the previous method. You can double-check the actual result by looking at the following tests:

[Test]
[DotNet7]
public void can_use_list_patterns_with_condition()
{
    Assert.That(SelfOrMessageWithCondition(new int[] { 0, 4, 3 }), Is.EqualTo("YEAH_3"));
    Assert.That(SelfOrMessageWithCondition(new int[] { 6, 4, 3 }), Is.EqualTo("error!"));
    Assert.That(SelfOrMessageWithCondition(new int[] { 1, 2, 3 }), Is.EqualTo("error!"));
    Assert.That(SelfOrMessageWithCondition(new int[] { 1, 6, 2, 3 }), Is.EqualTo("error!"));
    Assert.That(SelfOrMessageWithCondition(new int[] { 6, 3, 8, 4 }), Is.EqualTo("error!"));
}

To read more about List patterns, just head to the official documentation 🔗.

This article first appeared on Code4IT 🐧

Wrapping up

This is a new feature in C#. Have you ever used it in your production code?

Or is it “just” a nice functionality that nobody uses? Drop a message below if you have a real use if it 📩

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Happy coding!

🐧