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How to Resolve Error "CS0734: The /moduleassemblyname option may only be specified when building a target type of 'module'" in C#

The Compiler Error CS0734 is a build configuration error. The message reads: "The /moduleassemblyname option may only be specified when building a target type of 'module'".

In the .NET build process, the /moduleassemblyname compiler option is used specifically to allow a non-assembly file (a Module) to access internal types within the assembly it will eventually belong to. If you try to use this option while building a standard Assembly (.exe or .dll), the compiler raises this error because the option is irrelevant and invalid in that context—an assembly already knows its own name.

This guide explains the difference between Modules and Assemblies and how to fix your build configuration.

Understanding Modules vs. Assemblies​

  • Assembly (/target:library or /target:exe): A file (DLL or EXE) that contains a Manifest. It is the standard unit of deployment in .NET. It defines its own identity (Name, Version).
  • Module (/target:module): A file (.netmodule) that contains compiled code but no Assembly Manifest. It must be added to an Assembly later to be usable.

The option /moduleassemblyname:MyAssembly is used solely when compiling a Module. It tells the compiler: "I am a module. I don't have a home yet, but I promise I will eventually be part of an assembly named 'MyAssembly'. Please let me access 'internal' members of that assembly."

If you are compiling an Assembly (not a module), the file already has a manifest with a name. Specifying /moduleassemblyname is a contradiction.

Scenario 1: Command Line Build Mismatch​

This error is most frequently encountered when invoking the C# Compiler (csc.exe) manually or via a custom script.

Example of error: you are building a Library (DLL), but you included the module name switch.

# ⛔️ Error CS0734: You specified /target:library (default or explicit), 
# but also used /moduleassemblyname.
csc /target:library /moduleassemblyname:MyFinalAssembly Class1.cs

Scenario 2: MSBuild / .csproj Configuration​

In Visual Studio or .NET SDK projects, this can happen if you manually edit the .csproj file and add the <ModuleAssemblyName> property while the project OutputType is set to Library or Exe.

Example of error:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <!-- OutputType 'Library' means /target:library -->
    <OutputType>Library</OutputType>
    
    <!-- ⛔️ Error CS0734: This property is invalid for a Library -->
    <ModuleAssemblyName>MyFinalAssembly</ModuleAssemblyName>
  </PropertyGroup>
</Project>

Solution 1: Remove the Option (For Standard Builds)​

If your intention is to build a standard DLL or EXE application (which is true for 99% of C# development), simply remove the offending option. The assembly name is determined by the output filename or the <AssemblyName> property.

Command Line Fix​

# ✅ Correct: Just build the library.
csc /target:library /out:MyFinalAssembly.dll Class1.cs

.csproj Fix​

Remove the <ModuleAssemblyName> tag.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <OutputType>Library</OutputType>
    
    <!-- ✅ Correct: Defines the assembly name directly -->
    <AssemblyName>MyFinalAssembly</AssemblyName>
  </PropertyGroup>
</Project>

Solution 2: Change Target to Module​

If you actually intended to build a .netmodule (a specialized fragment of code to be linked later), you must change the compilation target.

Command Line Fix​

Change /target:library to /target:module.

# ✅ Correct: Now /moduleassemblyname is valid because we are building a module.
csc /target:module /moduleassemblyname:MyFinalAssembly Class1.cs

.csproj Fix​

Set the OutputType to Module.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    
    <!-- ✅ Correct: Sets /target:module -->
    <OutputType>Module</OutputType>
    <ModuleAssemblyName>MyFinalAssembly</ModuleAssemblyName>
  </PropertyGroup>
</Project>

Conclusion​

CS0734 ensures your build arguments are consistent.

  1. Check the Goal: Are you building a standard .dll/.exe?
    • Yes: Remove /moduleassemblyname (or the <ModuleAssemblyName> tag). Use <AssemblyName> instead.
  2. Check the Target: Are you building a .netmodule?
    • Yes: Ensure your target is set to /target:module (or <OutputType>Module</OutputType>).

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