What causes "Value cannot be null." in App-vNext/Polly?

Pass a non-null IAsyncCacheProvider, e.g. new MemoryCacheProvider(MemoryCache.Default).

App-vNext/Polly · error · ArgumentNullException

Value cannot be null.

Error message

Value cannot be null.

What it means

Thrown by CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, ...) (AsyncCacheTResultSyntax.cs:18) when cacheProvider is null. This is the typed-result convenience overload that wraps the provider via AsyncFor<TResult>() and builds a RelativeTtl; a null provider cannot be typed and would NRE, so it is rejected here.

Source

Thrown at src/Polly/Caching/AsyncCacheTResultSyntax.cs:22

public partial class Policy
{
    /// <summary>
    /// <para>Builds an <see cref="AsyncPolicy{TResult}"/> that will function like a result cache for delegate executions returning a <typeparamref name="TResult"/>.</para>
    /// <para>Before executing a delegate, checks whether the <paramref name="cacheProvider"/> holds a value for the cache key specified by <see cref="Context.OperationKey"/>.
    /// If the <paramref name="cacheProvider"/> provides a value from cache, 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>
    /// <typeparam name="TResult">The type of the result.</typeparam>
    /// <param name="cacheProvider">The cache provider.</param>
    /// <param name="ttl">Duration (ttl) for which to cache values.</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>
    public static AsyncCachePolicy<TResult> CacheAsync<TResult>(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
    {
        if (cacheProvider == null)
        {
            throw new ArgumentNullException(nameof(cacheProvider));
        }

        return CacheAsync(cacheProvider.AsyncFor<TResult>(), new RelativeTtl(ttl), DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
    }

    /// <summary>
    /// <para>Builds an <see cref="AsyncPolicy{TResult}"/> that will function like a result cache for delegate executions returning a <typeparamref name="TResult"/>.</para>
    /// <para>Before executing a delegate, checks whether the <paramref name="cacheProvider"/> holds a value for the cache key specified by <see cref="Context.OperationKey"/>.
    /// If the <paramref name="cacheProvider"/> provides a value from cache, 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>
    /// <typeparam name="TResult">The type of the result.</typeparam>
    /// <param name="cacheProvider">The cache provider.</param>
    /// <param name="ttlStrategy">A strategy for specifying ttl for values to be cached.</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

Pass a non-null IAsyncCacheProvider, e.g. new MemoryCacheProvider(MemoryCache.Default).
Confirm the DI registration and lifetime of the injected provider.
Construct the provider eagerly before building the policy.

Example fix

// before
var policy = Policy.CacheAsync<MyResult>(null, TimeSpan.FromMinutes(5));

// after
var policy = Policy.CacheAsync<MyResult>(new MemoryCacheProvider(MemoryCache.Default), TimeSpan.FromMinutes(5));

Defensive patterns

Strategy: validation

Validate before calling

ArgumentNullException.ThrowIfNull(cacheProvider);
var policy = Policy.CacheAsync<TResult>(cacheProvider, TimeSpan.FromMinutes(5), onCacheError);

Type guard

static bool IsCacheProviderValid(IAsyncCacheProvider? p) => p is not null;

Prevention

Register the IAsyncCacheProvider in DI and resolve it before building typed cache policies.
Enable nullable reference types to catch unassigned providers at compile time.
Construct the provider eagerly so it is never lazily null.

When it happens

Trigger: Calling Policy.CacheAsync<TResult>(null, TimeSpan.FromMinutes(5), onCacheError) — the IAsyncCacheProvider (1st positional) is null.

Common situations: DI did not register the cache provider; passing a field initialized after policy construction; provider creation threw and the variable stayed null.

Related errors

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/60fbe8ac1331d4bc. Report an issue: GitHub.