C# `ref` vs. `out`: When to Use Which Keyword?

Differences and application scenarios of ref and out keywords in C#

In C#, the ref and out keywords are used to pass objects by reference to methods or functions. They allow the caller to manipulate the object directly within the called method.

Usage scenarios:

ref and out both allow modification of the object passed to the method. The key difference is their initialization requirements:

ref:

• The caller must initialize the object before passing it to the method.
• Methods can access objects in both directions and modify their state.
• Any changes made within a method will be reflected in the caller's context.

out:

• The caller does not need to initialize the object before calling the method.
• Method initializes the object inside its body.
• Methods can only access objects in one direction and can assign values ​​to them.
• After the method is called, the caller will receive the initialized object.

Which keyword to choose:

Choose ref:

• The object is initialized before the method is called and needs to be modified.
• The original state of the object is important to the caller.

Choose out:

• Objects do not need to be initialized before method calls.
• Method should create a new object and return it.
• The original state of the object is not required by the caller.

Example:

Consider the following method of modifying the MyClass property of a Count object:

public void IncrementCount(ref MyClass myClass)
{
    myClass.Count++;
}

Passing a pre-initialized myClass object to this method will cause its Count properties to be modified. On the other hand, passing an empty or uninitialized object will result in a runtime error.

Use out in this scene:

public void CreateNewClass(out MyClass myClass)
{
    myClass = new MyClass();
}

In this case, the method creates and assigns a new myClass instance to the MyClass parameter. The new object is returned to the caller, which has an initialized object in its local context.

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