Why Does Calling ValidateOnStart Cause an Ambiguous Overload Error in C#?

This error means that the compiler sees two extension methods with the same signature—even though they look identical in source code—so it doesn’t know which one to choose. In your case, both candidates are

  Microsoft.Extensions.DependencyInjection.OptionsBuilderExtensions.ValidateOnStart(OptionsBuilder)

It often happens when your project ends up referencing two different versions (or assemblies) that both include the same extension method. For example, you might be getting one from a transitively referenced package (or assembly) and one directly because of how your Functions App and net7.0 environment pull in packages.

How to Fix

• Review Package References: Ensure you’re not referencing conflicting versions of packages such as Microsoft.Extensions.Options or Microsoft.Extensions.DependencyInjection. Check your project file and any transitive references to pick a single version.

• Use an Explicit Call: If you need to call a specific overload, you can fully qualify and cast appropriately so that the compiler can disambiguate (though fixing the references is generally the better solution).

For example, you might try something like:

CSHARPCopied
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;

namespace SomeNamespace
{
    internal static class ServiceCollectionExtensions
    {
        public static IServiceCollection AddServiceBusMessaging(this IServiceCollection serviceCollection)
        {
            // Cast or fully qualify to use the desired overload
            serviceCollection.AddOptions<SomeOptions>()
                .Configure<IConfiguration>((s, c) =>
                    c.GetSection("SectionName").Bind(s))
                .ValidateDataAnnotations()
                .AsOptionsBuilder()   // if you have an extension to help
                .ValidateOnStart();

            // Other registrations...
            return serviceCollection;
        }
    }
}

Note: The above snippet assumes you could disambiguate by obtaining the underlying OptionsBuilder instance in a specific way. Usually, the best solution is to remove the duplicate reference causing the ambiguous method.

Summary

Even though both definitions appear identical, something in the project’s references is causing them to be loaded twice. Check your NuGet packages and ensure that only one version of the assembly containing ValidateOnStart is available. This should resolve the CS0121 error.