Why Does C# Throw a 'Cannot Modify Return Value' Error When Modifying Auto-Implemented Properties?

Detailed explanation of "Cannot modify return value" error in C#

Auto-implemented properties in C# provide a convenient way to define properties, but sometimes when trying to modify the property value, it may result in a "Cannot modify return value" error.

Please see the following code:

public Point Origin { get; set; }

Origin.X = 10; // 产生 CS1612 错误

This code attempts to set the X coordinate of the Origin property. However, it will report an error saying that the return value of the property cannot be modified because it is not a variable.

Root cause: value types and reference types

This error occurs because Point is a value type (structure). Value types are copied by value, which means that when you assign a value type to a variable, a new copy is created. In this case, when you access the Origin property, you are actually accessing a copy of the Point value, not the original value stored in the class.

Solution

To resolve this issue, you have two options:

1. Use reference types: Change Point to a reference type (class) instead of a value type. Reference types are copied by reference, which means that when you assign a reference type to a variable, the pointer to the original object is copied. This allows the original object to be modified directly via its properties.
2. Stored in a temporary variable: If you want to keep Point as a value type, you need to store the result of the property access in a temporary variable before modifying it. For example:

Point temp = Origin;
temp.X = 10;
Origin = temp;

By storing the property value in the temp variable, you create a modifiable copy of the value type. You can then set properties to the modified copy.

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