== Why is this an issue?

https://en.wikipedia.org/wiki/Recursion[Recursion] is a technique used to define a problem in terms of the problem itself, usually in terms of a simpler version of the problem itself.

For example, the implementation of the generator for the n-th value of the https://en.wikipedia.org/wiki/Fibonacci_sequence[Fibonacci sequence] comes naturally from its mathematical definition, when recursion is used:

[source,csharp]
----
int NthFibonacciNumber(int n)
{
    if (n <= 1)
    {
        return 1;
    }
    else
    {
        return NthFibonacciNumber(n - 1) + NthFibonacciNumber(n - 2);
    }
}
----

As opposed to:

[source,csharp]
----
int NthFibonacciNumber(int n)
{
    int previous = 0;
	int last = 1;
	for (var i = 0; i < n; i++)
	{
        (previous, last) = (last, last + previous);
	}
	return last;
}
----

The use of recursion is acceptable in methods, like the one above, where you can break out of it. 

[source,csharp]
----
int NthFibonacciNumber(int n)
{
    if (n <= 1)
    {
        return 1; // Base case: stop the recursion
    }
    // ...
}
----

It is also acceptable and makes sense in some type definitions:

[source,csharp]
----
class Box : IComparable<Box>
{
    public int CompareTo(Box? other)
    {
        // Compare the two Box instances...
    }
}
----

With types, some invalid recursive definitions are caught by the compiler:

[source,csharp]
----
class C2<T> : C2<T>     // Error CS0146: Circular base type dependency
{
}

class C2<T> : C2<C2<T>> // Error CS0146: Circular base type dependency
{
}
----

In more complex scenarios, however, the code will compile but execution will result in a https://learn.microsoft.com/en-us/dotnet/api/system.typeloadexception[TypeLoadException] if you try to instantiate the class.

[source,csharp]
----
class C1<T>
{
}

class C2<T> : C1<C2<C2<T>>> // Noncompliant
{
}

var c2 = new C2<int>();     // This would result into a TypeLoadException
----

include::../resources.adoc[]

include::../rspecator.adoc[]