Why does ValidateOnStart trigger an ambiguous call error in my C# project?

The error

  CS0121: The call is ambiguous between the following methods or properties:
  "Microsoft.Extensions.DependencyInjection.OptionsBuilderExtensions.ValidateOnStart(Microsoft.Extensions.Options.OptionsBuilder)" and
  "Microsoft.Extensions.DependencyInjection.OptionsBuilderExtensions.ValidateOnStart(Microsoft.Extensions.Options.OptionsBuilder)"

indicates that the compiler has found two extension methods with identical signatures and it cannot choose one over the other. In practice, there’s no apparent difference between the two signatures, which usually means that two (or more) assemblies you’re referencing are both contributing the same extension method.

What’s Likely Happening

Even though the methods look identical in the error message, the root cause is that you have references (or transitive references) to different versions of the package(s) that provide the ValidateOnStart extension method. For example, if you reference more than one version of Microsoft.Extensions.Options or Microsoft.Extensions.DependencyInjection that include this extension, then you’ll end up with both on the compilation path. The compiler then sees two equally valid extension methods.

How to Resolve It

1. Review Package References:
   Check your project’s dependencies and transitive dependencies to see if you have two packages defining the same extension. Consolidate your package versions so that only one version is used. In your .csproj file, ensure you’re targeting a consistent version of Microsoft.Extensions.Options and Microsoft.Extensions.DependencyInjection.

2. Explicit Method Call (if needed):
   Although the best approach is to remove the duplicate reference, you can call the extension method as a static method on the type that defines it. This explicit call can sometimes help disambiguate which method you meant. For example:

   CSHARPCopied
   using Microsoft.Extensions.Options;
   
   // Instead of chaining the extension method...
   serviceCollection.AddOptions<SomeOptions>()
       .Configure<IConfiguration>((s, c) => c.GetSection("SectionName").Bind(s))
       .ValidateDataAnnotations()
       .ValidateOnStart();
   
   // ...call the method explicitly:
   var optionsBuilder = serviceCollection.AddOptions<SomeOptions>()
       .Configure<IConfiguration>((s, c) => c.GetSection("SectionName").Bind(s))
       .ValidateDataAnnotations();
   
   // Here you explicitly choose the one you want by referencing the static method on the extension type.
   Microsoft.Extensions.DependencyInjection.OptionsBuilderExtensions.ValidateOnStart(optionsBuilder);

   This ensures the compiler knows exactly which method you intend to use.

3. Clean and Rebuild:
   After adjusting your package references or the method call, do a clean and rebuild of your solution to ensure the old assembly references are cleared.

Summary

The ambiguity isn’t due to any difference in the method signature but due to duplicate method definitions coming from conflicting package versions. By aligning your package versions or choosing an explicit call, you can resolve the error CS0121.

If you have any further questions about how to manage these dependencies in a .NET isolated Function App targeting net7.0, feel free to ask.