How to Resolve Error "CS0013: Unexpected error writing metadata to file 'ModelStore.dll' - 'No logical space left to create more user strings'" in C#

The Compiler Error CS0013 is a rare but fatal file-format error. The message reads: "Unexpected error writing metadata to file 'MyAssembly.dll' -- 'No logical space left to create more user strings'".

This error indicates that you have hit a hard limit in the .NET Assembly file format. It does not mean your hard drive is full or your RAM is exhausted. Instead, it means the compilation result contains so many string literals (hardcoded text) that they exceed the maximum size allowed for the "User String Heap" in a single .dll or .exe (typically around 16MB of string data).

This usually happens in projects containing massive auto-generated code files or huge embedded data assets.

Understanding the Limit

A compiled .NET assembly consists of IL Code (instructions) and Metadata (definitions). The Metadata is divided into "Heaps":

• Blob Heap: Binary data.
• String Heap: Names of methods and classes.
• User String Heap: Hardcoded string literals used in your code (e.g., string s = "Hello World";).

The User String heap has a size limit defined by the ECMA-335 standard (historically constrained by 24-bit addressing within the metadata format). If your code contains enough hardcoded text to exceed this limit (~16MB), the compiler physically cannot write the .dll file, resulting in CS0013.

note

This error is almost never caused by human-written code. A human cannot type 16MB of text strings. This is invariably caused by code generators, tools, or copy-pasting huge datasets directly into C#.

Scenario 1: Massive String Literals (The most common cause)

The most frequent trigger is embedding a large JSON, XML, or HTML file directly into a C# string variable, or having a generated class with thousands of localized string fields.

Example of Problematic Code

Attempting to paste a 20MB text file into a variable:

public class BigData
{
    // ⛔️ Error: If this string is megabytes long, it fills the Metadata Heap.
    // The compiler tries to bake this text directly into the DLL header.
    public const string MyHugeJson = "{ ... [20MB of text] ... }";
}

Solution: Use External Resources

Do not compile data as code. Store the data in a separate file and load it at runtime, or use a Managed Resource (.resx).

Option A: Load from File (Runtime)

using System.IO;

public class BigData
{
    public static string MyHugeJson;

    static BigData()
    {
        // ✅ Correct: Keep the text out of the C# source code.
        // Ensure 'data.json' is set to "Copy to Output Directory" in properties.
        MyHugeJson = File.ReadAllText("data.json");
    }
}

Option B: Embedded Resource If you need the data inside the DLL, add it as an Embedded Resource. Resources are stored in a different section of the assembly that supports much larger limits (up to 2GB).

1. Add the file to your project.
2. Right-click > Properties.
3. Set Build Action to Embedded Resource.
4. Read it using Assembly.GetManifestResourceStream().

Scenario 2: Splitting Assemblies (The Architectural Fix)

If the strings are necessary code (e.g., a massive auto-generated ORM model or a GUI with 50,000 label definitions) and you cannot convert them to resources, you must split the project.

Since the limit applies per assembly (per .dll), dividing your code into two libraries doubles your available space.

Approach

1. Identify the bulk: Find the auto-generated files or the specific class files consuming the space.
2. Create a new Project: Create a Class Library (e.g., MyProject.Data.dll).
3. Move the files: Move half of the heavy code into the new library.
4. Reference: Reference the new library from the original project.

Before (One Project - Hits 16MB limit):

• MyApp.csproj
  • GeneratedCode_Part1.cs (10MB strings)
  • GeneratedCode_Part2.cs (10MB strings)
  • Total: 20MB -> CS0013

After (Two Projects - Safe):

• MyApp.Models.csproj
  • GeneratedCode_Part1.cs (10MB strings)
• MyApp.Main.csproj (References MyApp.Models)
  • GeneratedCode_Part2.cs (10MB strings)
  • Total: 10MB per DLL -> Success

Conclusion

CS0013 is a "Format Limit" error.

To resolve it:

1. Identify large strings: Look for huge hardcoded strings or massive auto-generated files.
2. Move data out of code: Use .txt, .json, or .resx files instead of string s = "...".
3. Divide and Conquer: If the code is essential, split your project into multiple smaller Class Libraries to spread the metadata load across multiple .dll files.