Unable to Load Type from Assembly in C Amazon Lambda Function: What It Is and How to Solve It

When developing applications in C

Unable to Load Type from Assembly in C# Amazon Lambda Function: What It Is and How to Solve It

When developing applications in C#, whether it’s for desktop, mobile, or cloud-based systems, we often encounter various errors. One such issue that has stumped many developers is the “Unable to load type from assembly” error in the context of Amazon Lambda Functions. In this article, we’ll explore what this error is, why it happens, and how to solve it.

What Is ‘Unable to Load Type from Assembly’ Error?

The error “Unable to load type from assembly” typically occurs when the CLR (Common Language Runtime) fails to find a specific type in an assembly that it’s trying to load. In the context of Amazon Lambda Functions, this can happen due to several reasons, such as:

  • The assembly does not contain the type specified.
  • The type is in a different assembly that has not been referenced.
  • There is a version mismatch between the assemblies.

How to Solve ‘Unable to Load Type from Assembly’ Error

Solving this error involves identifying the cause and correcting it accordingly. Here are the steps to debug and fix the issue:

Step 1: Check Your Assembly References

Ensure that you’ve correctly referenced the assembly containing the type. Use the fully qualified type name, which includes the namespace, to avoid conflicts with similar type names in different namespaces.

using MyNamespace.MyType;

Step 2: Verify Assembly Version

If you’re referencing an external assembly, ensure that the version of the assembly is compatible with your application. You can check the version of an assembly by right-clicking on the DLL file, navigating to ‘Properties’, and then to the ‘Details’ tab. If there’s a version mismatch, you might need to update or downgrade your assembly.

Step 3: Inspect Your AWS Lambda Function Handler

In a Lambda function, the function handler is the method in your code that processes events. When the handler is invoked, it receives an event object and a context object. Ensure that the handler is correctly defined, and it exists in the assembly.

public string FunctionHandler(string input, ILambdaContext context)
    return input?.ToUpper();

Step 4: Check for Build Errors

Ensure that your code builds successfully without any errors. Sometimes, the error could be due to an unsuccessful build.

dotnet build

Step 5: Verify the AWS Lambda .NET Core Deployment

When deploying your .NET Core application to AWS Lambda, ensure that all assemblies are correctly deployed. If you’re using the AWS Toolkit for Visual Studio or the AWS Lambda .NET Core Global Tool (dotnet lambda), it should take care of the deployment for you.

dotnet lambda deploy-function MyLambdaFunction


The “Unable to load type from assembly” error can be a roadblock, but it’s not insurmountable. By systematically checking your assembly references, verifying assembly versions, inspecting your AWS Lambda function handler, looking for build errors, and verifying your AWS Lambda .NET Core deployment, you can resolve this issue and get your application running smoothly.

Remember, every error is an opportunity to learn more about the system you’re working with. So, the next time you encounter this error, you’ll know exactly what to do. Happy coding!

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