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How to Resolve Error "CS0003: Out of memory" in C#

The Compiler Error CS0003 is a straightforward but critical error message: "Out of memory".

Unlike the common System.OutOfMemoryException which occurs while your application is running, CS0003 occurs while your application is being compiled. It means the C# compiler (csc.exe) has exhausted the system memory (RAM) available to it, or it has hit the virtual address space limit (typically 2GB in 32-bit processes) while attempting to process your source code.

This guide explores why the compiler runs out of memory and how to restructure your project or build environment to resolve it.

Understanding the Compiler Limits​

When you compile C# code, the compiler performs several memory-intensive tasks:

  1. Parsing: It reads text files and creates a Syntax Tree in memory.
  2. Symbol Binding: It resolves references between types and methods.
  3. Emission: It generates the Intermediate Language (IL).

If a single file is hundreds of megabytes in size, or if the logic requires an exponentially growing syntax tree, the compiler process can exceed its limits.

note

Context Matters: This error is rare in hand-written code. It almost exclusively appears when dealing with machine-generated code, massive static data arrays initialized inline, or legacy build environments restricted to 32-bit architecture.

Scenario 1: Massive Source Files and Auto-Generated Code​

The most common cause of CS0003 is a source file containing thousands (or millions) of lines of code, usually created by a tool (like a database ORM generator, a UI designer, or a static asset converter).

Problem: Inline Data Arrays​

Attempting to embed a large image or dataset directly into a C# array can crash the compiler.

// File: BigData.cs
// ⛔️ Error: Defining an array with millions of elements inline
// This forces the compiler to parse millions of integer tokens 
// and build a massive syntax tree.

public class BigData
{
    public static int[] MyHugeDataset = new int[] 
    { 
        1, 2, 3, 4, 5, 
        // ... imagine 10,000,000 more integers here ...
        9999999 
    };
}

Compiler Impact: The compiler tries to track every single integer as a syntax node, quickly consuming gigabytes of RAM.

Solution: Load Data from External Files​

Instead of hardcoding data, store it in a binary file, JSON, or Resource file (.resx), and load it at runtime.

// File: BigData.cs
// ✅ Correct: Load the data at runtime. The source code is now small.

using System.IO;
using System.Text.Json;

public class BigData
{
    public static int[] MyHugeDataset;

    static BigData()
    {
        // Load from a file copied to the output directory
        string json = File.ReadAllText("data.json");
        MyHugeDataset = JsonSerializer.Deserialize<int[]>(json);
    }
}

Scenario 2: Deeply Nested or Complex Expressions​

The compiler uses a recursive strategy to parse expressions. If an expression is too deep (e.g., thousands of chained string concatenations), the compiler's internal stack or heap may overflow.

Problem: String Concatenation Loops​

While this looks like simple code, if GenerateQuery() returns a string with 50,000 concatenated + operators, the compiler struggles to optimize the constant folding.

// ⛔️ Error: Extremely long chained expressions
string html = "<div>" + "<span>" + variable1 + "</span>" + ... + "</div>"; 
// If this chain goes on for thousands of lines, CS0003 can occur.

Solution: Use StringBuilder or Split Logic​

Break the expression into smaller statements or use StringBuilder.

// ✅ Correct: Using StringBuilder prevents the compiler from 
// having to track one massive expression tree.
var sb = new System.Text.StringBuilder();
sb.Append("<div>");
sb.Append("<span>");
sb.Append(variable1);
sb.Append("</span>");
// ...
string html = sb.ToString();

Scenario 3: 32-bit vs. 64-bit Build Environment​

Older versions of Visual Studio and MSBuild (or specific configurations in CI/CD pipelines) might default to using the 32-bit version of the C# compiler. A 32-bit process is strictly limited to 2GB (or sometimes 3GB or 4GB with large-address awareness) of memory, regardless of how much RAM your physical machine has (e.g., 32GB or 64GB).

If your project is large but not "broken," you might simply need a compiler with a higher ceiling.

How to Verify and Apply a Fix​

1. Check your .csproj file: Ensure you are not accidentally forcing a 32-bit processor architecture for the build process itself (this is different from the target platform of your app).

2. Use the 64-bit MSBuild: If you are building from the command line, ensure you are running the x64 version of the .NET SDK tools.

3. Visual Studio Settings: In Visual Studio 2022, the host process is 64-bit by default. However, in older versions (VS 2019 and earlier), you can configure the "Preferred Tool Architecture".

  • Go to Tools > Options.
  • Navigate to Projects and Solutions > Web Projects (or Build and Run).
  • Look for "Use the 64-bit version of IIS Express..." or similar settings depending on the project type.
tip

Modern .NET (Core) SDK builds usually default to using the dotnet process, which matches your OS architecture (likely x64). Ensure your CI/CD pipeline is not using an old MSBuild.exe from a 32-bit path (e.g., C:\Program Files (x86)\...).

Scenario 4: Infinite Loops in Source Generators​

With the introduction of Roslyn Source Generators in .NET 5+, it is possible to write code that generates other code during compilation.

If you write a Source Generator that contains a logic bug—specifically an infinite loop that keeps adding new syntax trees to the compilation context—the compiler will consume memory until it crashes with CS0003.

Debugging Source Generators​

If you suspect a Source Generator is the culprit:

  1. Disable Generators: Temporarily remove the reference to the Source Generator project.
  2. Check Build Process: If the build succeeds without the generator, the issue lies within the generator's logic.
  3. Review Loops: Look for while(true) or recursive calls in your ISourceGenerator.Execute implementation that lack a proper exit condition.

Conclusion​

The CS0003 error effectively means "The project is too big or too complex for the compiler to hold in its head at once".

To resolve it:

  1. Identify massive files: Look for auto-generated files or huge static arrays and move that data to external resources.
  2. Simplify expressions: Break down deeply nested logic or massive string concatenations.
  3. Upgrade the toolset: Ensure you are using a 64-bit build environment (Visual Studio 2022 or x64 .NET SDK) to access more than 2GB of RAM during compilation.

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