How Does C# Handle Passing Objects to Methods: Reference or Value?

In -depth discussion of object transmission in C#: reference and value

In C#, the transmission object to the method has always been a basic concept. It is generally believed that the non -original type is passed by reference. The changes made in the method will be retained, and the original value is passed by value, and a separate copy will be created.

However, an exception appears: when the System.Drawing.image object is passed to the method and the image is modified in this method, the original image object remains unchanged after returning the call party.

Reveal the truth: value transmission and reference transfer

To understand this phenomenon, we need to clarify the true nature of the object transmitted by the object in C#. The object itself will not be transmitted; on the contrary, their values ​​are transmitted as the initial parameter of the method. For the reference type, this value is a reference to the actual object, allowing changes to the underlying object in the call party.

The subtle difference between the value transmission and reference transfer

By default, C# uses "value transmission" to all types (including reference types). This means that the copy of the object is passed as the initial value of the parameter. Change the parameter value does not affect the original object. To change the attribute values ​​of the object inside the method, "reference transfer" can be used by specifying out or Ref keywords in the parameter statement. In this case, the parameter effectively references the same memory position as the parameter, and the changes to the parameters will be visible in the call.

sample code:

Conclusion The concepts of "value transmission" and "reference transfer" in

C# are different from its traditional understanding. In contrast, the values ​​of the parameter are transmitted, either as a copy (value type) or a reference (reference type). To change the attribute value of the object inside the method, you can use the OUT or REF keywords for reference and pass.

public void Foo(Image image) // 默认：值传递
{
    image = Image.FromStream(...); // 此更改调用方将看不到。
}

public void Foo(ref Image image) // 引用传递
{
    image = Image.FromStream(...); // 此更改对调用方可见。
}

public void Foo(Image image) // 对现有对象进行更改
{
    image.RotateFlip(...); // 此更改对调用方可见。
}

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