CodeQL documentation

Unsynchronized access to static collection member in non-static context

ID: cs/unsynchronized-static-access
Kind: problem
Severity: error
Precision: medium
   - concurrency
   - external/cwe/cwe-362
   - external/cwe/cwe-567
Query suites:
   - csharp-security-and-quality.qls

Click to see the query in the CodeQL repository

For performance reasons, most collection classes in the standard library are not thread-safe, instead requiring the user to guarantee they are used from at most one thread at a time by external locking or data structure invariants.

For example, the behavior of Dictionary when a write happens concurrently with another write or a read is undefined, and frequently leads to data corruption and can lead to ​ issues as serious as livelock.


If a static data member such as a Dictionary is likely to be accessed from multiple threads, ensure that either it is of a concurrency-safe collection type, or that all reads and writes are guarded by a suitable lock or monitor.


The following code uses a static dictionary to store properties, but provides unsynchronized access to that dictionary. This means that multiple threads can access the dictionary, potentially leading to a race condition.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Threading;

public class Configuration
    public static Dictionary<string, string> properties = new Dictionary<string, string>();

    // called concurrently elsewhere
    public string getProperty(string key)
        // BAD: unsynchronized access to static collection
        return dict["foo"];