Why Does 'Cannot Modify Return Value' Occur with Auto-Implemented Properties in C#?

"Cannot modify return value" error in C#

When using auto-implemented properties, a "Cannot modify return value" error may occur, for example:

<code class="language-csharp">public Point Origin { get; set; }

Origin.X = 10; // 出现 CS1612 错误</code>

Error cause analysis

This error occurs because Point is a value type (structure). When accessing the Origin attribute, a copy of the value held in the class is returned, not the value itself. Modifying a copy's properties does not affect the original value.

Solution

To solve this problem, the underlying value needs to be modified directly. For value types, this can be achieved by storing a copy in a temporary variable and setting a property on that variable:

<code class="language-csharp">Point originCopy = Origin;
originCopy.X = 10;
Origin = originCopy;</code>

Alternatively, you can create your own backup field to store the value type and modify the field directly:

<code class="language-csharp">private Point _origin;
public Point Origin
{
    get { return _origin; }
    set { _origin = value; }
}</code>

In this way, you can directly modify the Origin attribute:

<code class="language-csharp">Origin.X = 10; // 此行现在可以正常工作，不会报错</code>

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