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.

MSDN, C# Reference: Dictionary: Thread safety.