dotnetcore/CAP · error · ArgumentNullException

Value cannot be null. (Parameter 'configure')

Error message

Value cannot be null. (Parameter 'configure')

What it means

The delegate-based UseAzureServiceBus overload registers an AzureServiceBusOptionsExtension with the supplied configure action. A null delegate would leave the extension unable to configure anything, so ArgumentNullException is thrown up front.

Solutions

1. Pass a real configuration lambda: options.UseAzureServiceBus(opt => opt.ConnectionString = cs)
2. If only a connection string is needed, use the string overload UseAzureServiceBus(connectionString)
3. Ensure the delegate variable is assigned before registration; assert with ArgumentNullException.ThrowIfNull in your own helper
4. Remove the AddAzureServiceBus call entirely if Service Bus is not used, rather than calling it with null

Example fix

// before
Action<AzureServiceBusOptions>? cfg = BuildConfigurator(); // null
options.UseAzureServiceBus(cfg!);
// after
options.UseAzureServiceBus(opt =>
{
    opt.ConnectionString = cs;
    opt.AutoProvision = true;
});

if (configure is null) throw new InvalidOperationException("UseAzureServiceBus requires a configure delegate");

bool HasConfigure(Action<AzureServiceBusOptions>? a) => a is not null;

try { options.UseAzureServiceBus(configure); } catch (ArgumentNullException ex) when (ex.ParamName == "configure") { logger.LogError(ex, "ASB configure delegate was null"); throw; }

When it happens

Trigger: Calling options.UseAzureServiceBus((Action<AzureServiceBusOptions>)null) — e.g. a delegate variable loaded from a factory/config hook that returned null, or a generic helper that passes the action through without initializing it.

Common situations: Convention-based startup helpers that resolve the configure callback from DI and get null; test setup that passes a placeholder null action; refactors that removed the lambda but kept the call.

Related errors

• Value cannot be null. (Parameter 'configure')
• Value cannot be null. (Parameter 'options')
• Value cannot be null. (Parameter 'topics')
• Value cannot be null. (Parameter 'connectionString')
• Value cannot be null. (Parameter 'topicNames')

AI-assisted analysis of dotnetcore/CAP@e52b8508e5 (2026-09-14). Data as JSON: /api/errors/cae0050d0272cbbb. Report an issue: GitHub.

Appendix: source

Thrown at src/DotNetCore.CAP.AzureServiceBus/CAP.Options.Extensions.cs:31

    /// Configuration to use Azure Service Bus in CAP.
    /// </summary>
    /// <param name="options">CAP configuration options</param>
    /// <param name="connectionString">Connection string for namespace or the entity.</param>
    public static CapOptions UseAzureServiceBus(this CapOptions options, string connectionString)
    {
        if (connectionString == null) throw new ArgumentNullException(nameof(connectionString));

        return options.UseAzureServiceBus(opt => { opt.ConnectionString = connectionString; });
    }

    /// <summary>
    /// Configuration to use Azure Service Bus in CAP.
    /// </summary>
    /// <param name="options">CAP configuration options</param>
    /// <param name="configure">Provides programmatic configuration for the Azure Service Bus.</param>
    public static CapOptions UseAzureServiceBus(this CapOptions options, Action<AzureServiceBusOptions> configure)
    {
        if (configure == null) throw new ArgumentNullException(nameof(configure));

        options.RegisterExtension(new AzureServiceBusOptionsExtension(configure));

        return options;
    }
}

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