c# – 为什么IL将此值设置两次？

using System;
using System.Linq;
using System.Collections.Generic;
using Microsoft.CSharp;

public class C {

    public C()
    {
        x = 4;
    }

    public int x { get; } = 5;
}

它给了我这个代码：

using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;
[assembly: AssemblyVersion("0.0.0.0")]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.Default | DebuggableAttribute.DebuggingModes.DisableOptimizations | DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints | DebuggableAttribute.DebuggingModes.EnableEditAndContinue)]
[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum,SkipVerification = true)]
[module: UnverifiableCode]
public class C
{
    [DebuggerBrowsable(DebuggerBrowsableState.Never),CompilerGenerated]
    private readonly int <x>k__BackingField;
    public int x
    {
        [CompilerGenerated]
        get
        {
            return this.<x>k__BackingField;
        }
    }
    public C()
    {
        this.<x>k__BackingField = 5;
        base(); // This is not valid C#,but it represents the IL correctly.

        this.<x>k__BackingField = 4;
    }
}

我没有得到的是为什么它会在构造函数中两次分配支持字段：

this.<x>k__BackingField = 5;
        base(); // This is not valid C#,but it represents the IL correctly.

        this.<x>k__BackingField = 4;.

这是网站的错误还是Roslyn编译器实际上是这样做的(真是愚蠢的imo)？

我的意思是,如果我这样做

public C(int x)
 {
   this.x = x;
 }

public int x { get; } = 5;

并创建该代码：

public C(int x)
{
    this.<x>k__BackingField = 5;
    base(); // This is not valid C#,but it represents the IL correctly.

    this.<x>k__BackingField = x;
}

但它不应该优化出来吗？