microsoft/aspire · error · ArgumentNullException

Value cannot be null. (Parameter 'config')

Error message

Value cannot be null. (Parameter 'config')

What it means

AksNodePoolResource is a primary-constructor class whose Config property initializer throws ArgumentNullException when the 'config' parameter is null. The library treats the Azure-specific node pool configuration as mandatory because it is used to generate the AKS agent pool Bicep profile; constructing the resource without it is a programming error, so it fails fast at construction time.

Solutions

1. Construct a valid AksNodePoolConfig (e.g. with VM size and node count) and pass it instead of null
2. Check the argument order at the call site — null may be intended for a different parameter
3. If config comes from a factory/variable, null-check or use the null-forgiving-free pattern '?? throw new ArgumentNullException' before constructing
4. If you believe null config should be legal, file an issue; otherwise wrap construction in try/catch ArgumentNullException to surface a clearer message

Example fix

// before
var pool = new AksNodePoolResource("system", null!, aksEnv);
// after
var config = new AksNodePoolConfig { VmSize = "Standard_D4s_v5", NodeCount = 3 };
var pool = new AksNodePoolResource("system", config, aksEnv);

if (config is null)
{
    throw new ArgumentNullException(nameof(config), "AKS node pool requires a non-null AksNodePoolConfig (vmSize, node count, autoscaling settings).");
}
var pool = new AksNodePoolResource(name, config, aksEnv);

if (config is not AksNodePoolConfig validConfig)
{
    throw new ArgumentException("Expected a non-null AksNodePoolConfig.", nameof(config));
}

try
{
    var pool = new AksNodePoolResource(name, config, aksEnv);
}
catch (ArgumentNullException ex) when (ex.ParamName == "config")
{
    // Fail with a clearer app-model error pointing at the resource name.
    throw new InvalidOperationException($"Node pool '{name}' was created without an AksNodePoolConfig.", ex);
}

When it happens

Trigger: Calling the AksNodePoolResource constructor (directly or via a hosting API like AddAksNodePool) with a null AksNodePoolConfig argument, e.g. new AksNodePoolResource("pool", null!, env) or passing an uninitialized config variable/return value of a factory that returned null.

Common situations: Passing the result of a config-builder method that can return null; misordered constructor arguments so null lands in the config slot; conditional config creation ('AksNodePoolConfig? cfg = condition ? Build() : null') then using cfg without a null check; C# nullable warnings suppressed with null! or #pragma.

Related errors

• Object value cannot be null (ArgumentNullException: parent)
• Object value cannot be null (ArgumentNullException: parent)
• Value cannot be null. (Parameter 'parent')
• Value cannot be null. (Parameter 'parent')
• Value cannot be null. (Parameter 'parent')

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/371cec2edc80ad9b. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.Kubernetes/AksNodePoolResource.cs:29

/// that is used to generate Azure Bicep for the AKS agent pool profile.
/// </summary>
/// <param name="name">The name of the node pool resource.</param>
/// <param name="config">The Azure-specific node pool configuration.</param>
/// <param name="parent">The parent AKS environment resource.</param>
public class AksNodePoolResource(
    string name,
    AksNodePoolConfig config,
    AzureKubernetesEnvironmentResource parent) : KubernetesNodePoolResource(name, parent.KubernetesEnvironment)
{
    /// <summary>
    /// Gets the parent AKS environment resource.
    /// </summary>
    public AzureKubernetesEnvironmentResource AksParent { get; } = parent ?? throw new ArgumentNullException(nameof(parent));

    /// <summary>
    /// Gets the Azure-specific node pool configuration.
    /// </summary>
    public AksNodePoolConfig Config { get; } = config ?? throw new ArgumentNullException(nameof(config));
}

View on GitHub (pinned to 25830f84bd)