Are C# Arrays and Lists Passed by Reference or by Value?

Are C# arrays and lists passed by reference by default?

Arrays and lists are essential data structures in C#, and understanding their behavior when passed to functions is critical to efficient coding.

Are they passed by reference?

No, arrays and lists in C# are not passed by reference by default. Instead, their references are passed by value. This means that any modifications made to the contents of the array or list will be visible to the caller, but reassignments to the array or list itself will not be reflected back to the caller.

Why not pass by reference?

Passing an array or list by reference can significantly slow down program execution because a new copy of the entire array or list needs to be created each time the function is called. This overhead will be especially noticeable for large data sets.

When to pass by reference

In rare cases, if you want the caller to see changes to the content and the reference itself, you can explicitly pass the reference by reference using the ref modifier. However, this should be done with caution as it can introduce complexities and potential errors.

Example

Consider the following code:

void Foo(int[] data) {
    data[0] = 1; // 调用者可以看到此更改
}
void Bar(int[] data) {
    data = new int[20]; // 但看不到此更改
}

In the first function Foo, any changes made to the contents of the array will be visible to the caller because the reference to the array is passed by value. However, in the second function Bar, the reassignment of the array is not visible to the caller because the reference itself is passed by value.

Conclusion

In most cases, having arrays and lists passed by value will provide the best performance and reduce the risk of introducing errors. Passing by reference should only be done explicitly when necessary using the ref modifier.

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