App-vNext/Polly · error · ArgumentNullException

Value cannot be null. (Parameter 'onCacheGet')

Error message

Value cannot be null. (Parameter 'onCacheGet')

What it means

Thrown by the terminal CacheAsync<TResult>(...) overload when onCacheGet is null. This overload requires explicit lifecycle callbacks (they are not optional here); a null onCacheGet would cause a NullReferenceException at cache-hit time, so the guard rejects it at construction. Provider, TTL, and key-strategy guards run before it.

Source

Thrown at src/Polly/Caching/AsyncCacheTResultSyntax.cs:996

    {
        if (cacheProvider == null)
        {
            throw new ArgumentNullException(nameof(cacheProvider));
        }

        if (ttlStrategy == null)
        {
            throw new ArgumentNullException(nameof(ttlStrategy));
        }

        if (cacheKeyStrategy == null)
        {
            throw new ArgumentNullException(nameof(cacheKeyStrategy));
        }

        if (onCacheGet == null)
        {
            throw new ArgumentNullException(nameof(onCacheGet));
        }

        if (onCacheMiss == null)
        {
            throw new ArgumentNullException(nameof(onCacheMiss));
        }

        if (onCachePut == null)
        {
            throw new ArgumentNullException(nameof(onCachePut));
        }

        return new AsyncCachePolicy<TResult>(cacheProvider, ttlStrategy, cacheKeyStrategy, onCacheGet, onCacheMiss, onCachePut, onCacheGetError, onCachePutError);
    }

    private static void EmptyCallback(Context context, string key)
    {
        // No-op

View on GitHub (pinned to d0e46bdb1e)

Solutions

1. Provide a non-null Action<Context,string> for onCacheGet, even if it is a no-op like (_, _) => { }.
2. Prefer the simpler overloads that supply EmptyCallback automatically when you do not need lifecycle hooks.
3. Null-check the callbacks at the call site.

Example fix

// before
var policy = Policy.CacheAsync<string>(provider, ttlStrategy, keyFunc, null, onMiss, onPut, null, null);
// after
Action<Context, string> noop = (_, _) => { };
var policy = Policy.CacheAsync<string>(provider, ttlStrategy, keyFunc, noop, onMiss, onPut, null, null);

Action<Context, string> onGet = onCacheGet ?? ((_, _) => { });
var policy = Policy.CacheAsync<TResult>(provider, ttlStrategy, keyFunc, onGet, onMiss, onPut, null, null);

static bool HasCallback(Action<Context, string>? a) => a is not null;

try { var p = Policy.CacheAsync<R>(provider, ttl, keyFunc, onGet, onMiss, onPut, null, null); }
catch (ArgumentNullException ex) when (ex.ParamName == "onCacheGet") { /* supply a no-op callback */ }

When it happens

Trigger: Calling the full overload with null for the onCacheGet Action<Context,string>, or passing a callbacks object whose onCacheGet field is null.

Common situations: Developer assumed the callbacks were optional (they are in some overloads via EmptyCallback, but not when you call the full overload directly); refactor removed the callback assignment; test code passed null placeholders.

Related errors

• Value cannot be null. (Parameter 'onCacheMiss')
• Value cannot be null. (Parameter 'onCachePut')
• Value cannot be null. (Parameter 'ttlStrategy')
• Value cannot be null. (Parameter 'wrappedCacheProvider')
• Value cannot be null. (Parameter 'action')

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/3922479d592dc868. Report an issue: GitHub.