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How to Resolve Error "CS0656: Missing compiler required member 'object.member'" in C#

The Compiler Error CS0656 is a dependency or environment configuration error. The message reads: "Missing compiler required member 'System.Runtime.CompilerServices.Unsafe.Add' (or similar member)".

This error occurs when the C# compiler tries to generate code for a specific language feature (like dynamic, foreach, async/await, or modern pattern matching), but it cannot find the underlying .NET types or methods required to implement that feature in the standard library references provided. It essentially means your project is trying to use a C# feature that your referenced .NET Framework/Core version doesn't fully support, or you are missing a critical NuGet package.

This guide explains why these members go missing and how to restore them.

Understanding "Compiler Required Members"​

Many C# keywords are "Syntactic Sugar." When you write them, the compiler transforms them into complex method calls.

  • dynamic $\rightarrow$ compiles to calls inside Microsoft.CSharp.dll.
  • (a, b) (Tuples) $\rightarrow$ compiles to System.ValueTuple.
  • Span<T> access $\rightarrow$ implies System.Runtime.CompilerServices.Unsafe.

If you use the feature, the compiler looks for the specific helper method in your referenced assemblies. If the assembly is missing or outdated, the compiler cannot generate the final IL code, resulting in CS0656.

Scenario 1: Missing Microsoft.CSharp (Dynamic Keyword)​

This is a classic occurrence. You try to use the dynamic keyword in a project (often a Console App or Class Library) that does not reference the specific assembly required for dynamic binding.

Example of error​

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0656: Missing compiler required member 
        // 'Microsoft.CSharp.RuntimeBinder.Binder.Convert'
        dynamic data = GetDynamicObject();
        Console.WriteLine(data.Name);
    }

    static object GetDynamicObject() => new { Name = "Test" };
}

Solution: Add Reference​

You need to add a reference to Microsoft.CSharp.

For .NET Framework (Legacy):

  1. Right-click References in Solution Explorer.
  2. Select Add Reference.
  3. Search for Microsoft.CSharp and check it.

For .NET Core / .NET 5+: This is usually included by default. If missing, check if you manually excluded it in the .csproj.

<ItemGroup>
  <!-- Ensure this is NOT excluded or missing if you need dynamic -->
  <Reference Include="Microsoft.CSharp" />
</ItemGroup>

Scenario 2: Versions Mismatch (System.Runtime / NetStandard)​

This often happens when referencing a .NET Standard 2.0 library from an old .NET Framework 4.6.1 project, or when there is a conflict between NuGet packages versions. The compiler might look for a member like System.Runtime.CompilerServices.Unsafe.Add, which exists in newer runtimes but not in the older ones loaded by your project.

Example of error​

Using modern Span<T> or high-performance memory features without the supporting NuGet package.

// ⛔️ Error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.Unsafe.Add'
Span<int> slice = myArray.AsSpan().Slice(0, 10);

Solution: Install System.Runtime.CompilerServices.Unsafe​

You likely need to install or update a specific NuGet package to bridge the gap.

  1. Open NuGet Package Manager.
  2. Search for System.Runtime.CompilerServices.Unsafe.
  3. Install the latest stable version.

This provides the missing "glue" code that the compiler is looking for.

Scenario 3: Missing System.ValueTuple (Legacy Frameworks)​

If you use C# 7.0+ Tuple syntax (e.g., (int x, int y) = GetCoords();) in a project targeting .NET Framework 4.6.2 or older, the compiler will fail. The ValueTuple type was not built into mscorlib in those versions.

Example of error​

public (int, int) GetCoordinates()
{
    // ⛔️ Error CS0656: Missing compiler required member 'System.ValueTuple`2.Create'
    return (10, 20);
}

Solution: Install NuGet Package​

Install the System.ValueTuple NuGet package.

  1. Right-click Project > Manage NuGet Packages.
  2. Install System.ValueTuple.

Conclusion​

CS0656 means your Tooling (C# Compiler) is newer than your Libraries (References).

  1. Identify the Missing Member: Read the error message carefully. Is it looking for Binder, ValueTuple, or Unsafe?
  2. Add References:
    • Microsoft.CSharp for dynamic issues.
    • System.ValueTuple for Tuple issues in old frameworks.
    • System.Runtime.CompilerServices.Unsafe for Span/Memory issues.
  3. Check Targeting: Ensure your project targets a version of .NET that supports the features you are using, or install the "polyfill" NuGet packages.

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