microsoft/aspire · error · InvalidOperationException

' ' is missing AppendValueProvider(object, string).

Error message

'{builder.GetType().FullName}' is missing AppendValueProvider(object, string).

What it means

AppendValueProvider reflectively requires a public instance AppendValueProvider(object, string) method on the builder. When reflection cannot find that exact signature, this InvalidOperationException is thrown with the builder's type name.

Solutions

Pass the ReferenceExpression.Builder instance (un-built) as the builder argument.
Align Aspire.Hosting versions so Builder.AppendValueProvider(object, string) exists.
If publishing trimmed, add a trimmer root/DynamicDependency for ReferenceExpression.Builder.
Check the referenced Aspire.Hosting README/changelog for signature changes to AppendValueProvider.

Example fix

// before
AppendValueProvider(builtExpression, valueProvider); // built expression lacks Builder methods

// after
var builder = CreateReferenceExpressionBuilder();
AppendValueProvider(builder, valueProvider);

Defensive patterns

Strategy: type-guard

Validate before calling

var m = builder.GetType().GetMethod("AppendValueProvider", BindingFlags.Instance | BindingFlags.Public, null, [typeof(object), typeof(string)], null);
if (m is null)
    throw new NotSupportedException($"{builder.GetType().FullName} lacks AppendValueProvider(object, string); check Aspire.Hosting version.");

Type guard

static bool SupportsAppendValueProvider(object builder) =>
    builder.GetType().GetMethod("AppendValueProvider", BindingFlags.Instance | BindingFlags.Public, null, [typeof(object), typeof(string)], null) is not null;

Try / catch

try
{
    AppendValueProvider(builder, valueProvider);
}
catch (InvalidOperationException ex) when (ex.Message.Contains("missing AppendValueProvider"))
{
    logger.LogError(ex, "Builder surface mismatch on {Type}", builder.GetType().FullName);
}

Prevention

Verify the value provider is appended on an un-built ReferenceExpression.Builder.
Read the Aspire.Hosting changelog when bumping versions — AppendValueProvider signatures have evolved.
Keep integration tests covering value-provider interpolation paths.
Add trimming annotations for the builder methods in AOT scenarios.

When it happens

Trigger: ToValueReferenceExpression calls AppendValueProvider(builder, valueProvider) and GetMethod("AppendValueProvider", Public|Instance, [typeof(object), typeof(string)]) returns null.

Common situations: Aspire.Hosting version change altered the AppendValueProvider signature or overloads; wrong object passed as builder; trimming stripped the method in AOT-published apps.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/b87909ba32ccf5f6. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.RemoteHost/Ats/ReferenceExpressionRef.cs:347

            "AppendLiteral",
            BindingFlags.Instance | BindingFlags.Public,
            binder: null,
            [typeof(string)],
            modifiers: null)
            ?? throw new InvalidOperationException($"'{builder.GetType().FullName}' is missing AppendLiteral(string).");

        appendLiteralMethod.Invoke(builder, [value]);
    }

    private static void AppendValueProvider(object builder, object valueProvider)
    {
        var appendValueProviderMethod = builder.GetType().GetMethod(
            "AppendValueProvider",
            BindingFlags.Instance | BindingFlags.Public,
            binder: null,
            [typeof(object), typeof(string)],
            modifiers: null)
            ?? throw new InvalidOperationException($"'{builder.GetType().FullName}' is missing AppendValueProvider(object, string).");

        appendValueProviderMethod.Invoke(builder, [valueProvider, null]);
    }

    private static object BuildReferenceExpression(object builder)
    {
        var buildMethod = builder.GetType().GetMethod("Build", BindingFlags.Instance | BindingFlags.Public)
            ?? throw new InvalidOperationException($"'{builder.GetType().FullName}' is missing Build().");

        return buildMethod.Invoke(builder, null)
            ?? throw new InvalidOperationException($"'{builder.GetType().FullName}.Build()' returned null.");
    }

    private static object CreateConditionalReferenceExpression(
        object condition,
        string matchValue,
        object whenTrue,
        object whenFalse)

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