When should a member or class be static in C#, and what problems can mutable static state cause in an API?

Static members versus instance state in C#

Uses static members for type-wide stateless behavior and constants while avoiding hidden global mutable state and lifetime problems.

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I use static members for type-wide behavior that needs no instance identity, mutable state, or replaceable dependency. Math-like functions and immutable shared metadata are reasonable examples. Customer-specific balance or request context belongs on an instance. A mutable static dictionary is shared by every request in that process, needs synchronization, survives between tests, and disappears on restart. It also differs per replica, so it cannot represent globally authoritative state.

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Type-wide behavior, instance identity, pure helpers, global state, concurrency, tests, configuration, caching, and process boundaries.

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A static member belongs to the type rather than one object instance. It fits constants, pure helpers, and behavior that needs no instance state or replaceable dependency; mutable static state is global state inside the process.
I use static members for type-wide behavior that needs no instance identity, mutable state, or replaceable dependency. Math-like functions and immutable shared metadata are reasonable examples. Customer-specific balance or request context belongs on an instance. A mutable static dictionary is shared by every request in that process, needs synchronization, survives between tests, and disappears on restart. It also differs per replica, so it cannot represent globally authoritative state.
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Static methods can be easy to call but hard to replace when they hide time, randomness, filesystem, or network access. I inject those capabilities behind focused contracts where tests or runtime selection matter. A static cache may be appropriate for immutable bounded data with explicit invalidation, but IMemoryCache or a managed singleton usually makes lifetime and limits clearer. Singleton dependency injection is still one instance per container and can participate in composition.

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Using static to avoid dependency injection.
Storing request or tenant data in static fields.
Treating static state as shared across replicas.
Leaving a static cache unbounded.

When should a member or class be static in C#, and what problems can mutable static state cause in an API?

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