How to Resolve Error "CS0573: 'Field': cannot have instance field initializers in structs" in C#

The Compiler Error CS0573 is a restriction related to Structs in older versions of C#. The message reads: " 'Field': cannot have instance field initializers in structs".

In C#, a struct is a value type. Historically (prior to C# 10), structs were required to perform all initialization inside a constructor to ensure performance predictability. This meant you could not assign a default value directly where the field was declared (e.g., public int x = 10;).

While C# 10 and C# 11 have relaxed these rules significantly, you will still encounter this error if you are using an older language version (like C# 9.0 in .NET 5 or .NET Framework 4.8) or if you have specific layout modifiers.

This guide explains the restriction and how to initialize struct fields correctly.

Understanding the Struct Limitation

Unlike classes, structs are value types that are often created without calling a constructor (e.g., creating an array new MyStruct[10] creates 10 structs initialized to zero/null).

If C# allowed inline initializers (int x = 5) in older versions, the runtime would have to execute that initialization logic every time a struct was created, which contradicts the "zero-overhead" default behavior of value types. Therefore, the compiler historically enforced that fields must either start at 0/null or be set explicitly in a constructor you define.

Scenario: Inline Initialization in Older C#

This error commonly occurs when a developer treats a struct exactly like a class.

Example of error:

public struct Point
{
    // ⛔️ Error CS0573: 'Point.X': cannot have instance field initializers in structs
    public int X = 10; 
    
    // ⛔️ Error CS0573
    public int Y = 20;
}

Solution 1: Use a Constructor (The Standard Fix)

If you are on an older version of .NET (Framework, Core 3.1, or .NET 5), you must move the initialization logic into a parameterized constructor.

Solution:

public struct Point
{
    public int X;
    public int Y;

    // ✅ Correct: Initialize fields inside the constructor.
    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
}

// Usage:
// var p = new Point(10, 20);

note

Default Constructor: Prior to C# 10, you cannot define a parameterless constructor public Point() { ... } in a struct. You must rely on the default (zeroed) state or use a factory method.

Solution 2: Upgrade to C# 10+

If you are using .NET 6 or newer (which defaults to C# 10), the language rules have changed. Inline field initializers for structs are allowed.

If you are seeing this error in a modern project, check your .csproj file. You might be inadvertently forcing an old language version.

How to Check Language Version

Open your .csproj file and check for <LangVersion>.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    
    <!-- ⛔️ Problem: This forces C# 9 rules, where field init is illegal -->
    <LangVersion>9.0</LangVersion> 
  </PropertyGroup>
</Project>

Solution

Remove the LangVersion tag (to use the SDK default) or set it to 10.0 or higher.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    <!-- ✅ Correct: C# 10+ supports struct field initializers -->
    <LangVersion>10.0</LangVersion> 
  </PropertyGroup>
</Project>

After upgrading:

public struct Point
{
    // ✅ Valid in C# 10+
    public int X = 10; 
    public int Y = 20;
}

Conclusion

CS0573 is primarily a versioning restriction.

1. Check your Version: Are you using .NET Framework or an old C# version?
2. The Code Fix: Move assignments from the field declaration (int x = 10;) to a constructor.
3. The Project Fix: If on .NET 6+, upgrade your language version to C# 10 to allow this syntax.