App-vNext/Polly · error · ArgumentNullException

Value cannot be null.

Error message

Value cannot be null.

What it means

Thrown as ArgumentNullException(nameof(cacheKeyStrategy)) from the CacheAsync overload at AsyncCacheSyntax.cs:52 that accepts an ICacheKeyStrategy. This overload wraps the strategy's GetCacheKey method into a Func<Context, string> and delegates to another overload. The guard at line 50–53 fires before the delegation, ensuring the key strategy is present.

Source

Thrown at src/Polly/Caching/AsyncCacheSyntax.cs:52

    /// <summary>
    /// <para>Builds an <see cref="AsyncPolicy"/> that will function like a result cache for delegate executions returning a result.</para>
    /// <para>Before executing a delegate returning a result, checks whether the <paramref name="cacheProvider"/> holds a value for the cache key determined by applying the <paramref name="cacheKeyStrategy"/> to the execution <see cref="Context"/>.
    /// If the <paramref name="cacheProvider"/> provides a value, returns that value and does not execute the governed delegate.  If the <paramref name="cacheProvider"/> does not provide a value, executes the governed delegate, stores the value with the <paramref name="cacheProvider"/>, then returns the value.
    /// </para>
    /// </summary>
    /// <param name="cacheProvider">The cache provider.</param>
    /// <param name="ttl">Duration (ttl) for which to cache values.</param>
    /// <param name="cacheKeyStrategy">The cache key strategy.</param>
    /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
    /// <returns>The policy instance.</returns>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="cacheProvider"/> is <see langword="null"/>.</exception>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="cacheKeyStrategy"/> is <see langword="null"/>.</exception>
    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
    {
        if (cacheKeyStrategy is null)
        {
            throw new ArgumentNullException(nameof(cacheKeyStrategy));
        }

        return CacheAsync(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy.GetCacheKey, onCacheError);
    }

    /// <summary>
    /// <para>Builds an <see cref="AsyncPolicy" /> that will function like a result cache for delegate executions returning a result.</para>
    /// <para>Before executing a delegate returning a result, checks whether the <paramref name="cacheProvider" /> holds a value for the cache key determined by applying the <paramref name="cacheKeyStrategy"/> to the execution <see cref="Context"/>.
    /// If the <paramref name="cacheProvider" /> provides a value, returns that value and does not execute the governed delegate.  If the <paramref name="cacheProvider" /> does not provide a value, executes the governed delegate, stores the value with the <paramref name="cacheProvider" />, then returns the value.
    /// </para>
    /// </summary>
    /// <param name="cacheProvider">The cache provider.</param>
    /// <param name="ttlStrategy">A strategy for specifying ttl for values to be cached.</param>
    /// <param name="cacheKeyStrategy">The cache key strategy.</param>
    /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
    /// <returns>The policy instance.</returns>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="cacheProvider"/> is <see langword="null"/>.</exception>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="ttlStrategy"/> is <see langword="null"/>.</exception>

View on GitHub (pinned to d0e46bdb1e)

Solutions

1. Provide a non-null ICacheKeyStrategy implementation, e.g. a DefaultCacheKeyStrategy subclass
2. Use the Func<Context, string> overload if a full ICacheKeyStrategy class is unnecessary
3. Ensure DI registration for the strategy resolves to a non-null instance

Example fix

// before
var policy = Policy.CacheAsync(provider, TimeSpan.FromMinutes(5), (ICacheKeyStrategy)null); // throws

// after
var policy = Policy.CacheAsync(provider, TimeSpan.FromMinutes(5), new ContextualTtl());
// or use the Func overload:
var policy = Policy.CacheAsync(provider, TimeSpan.FromMinutes(5), ctx => ctx.OperationKey);

if (cacheKeyStrategy is null)
{
    throw new InvalidOperationException("Cache key strategy is required.");
}
var policy = Policy.CacheAsync(provider, ttl, cacheKeyStrategy);

public static bool IsNonNullStrategy(ICacheKeyStrategy strategy) => strategy is not null;

When it happens

Trigger: Calling Policy.CacheAsync(cacheProvider, ttl, (ICacheKeyStrategy)null) — passing null as the ICacheKeyStrategy parameter. The guard fires before the RelativeTtl and GetCacheKey wrapping occurs.

Common situations: Calling the overload that takes ICacheKeyStrategy without providing a strategy (use the overload with Func<Context,string> instead). DI container returning null for the strategy registration. Migrating between cache key strategy types and forgetting to pass the new instance.

Related errors

• Value cannot be null.
• Value cannot be null.
• Value must be greater than zero.
• Value must be greater than or equal to zero.
• Value must be greater than zero.

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/a18199a985971710. Report an issue: GitHub.