We utilize async & await in C# whenever an I/O call occurs within our functions to prevent blocking the calling threads. This is achieved by returning a Task (or Task<T>) that represents a promise for future completion from that function.

However, there may be scenarios where the function rarely initiates an I/O call, and most of the time, it retrieves the value directly from local memory. Therefore, if the function already possesses the value upon returning to the caller, it would be beneficial to return the value itself rather than the Task. Returning the Task might result in unnecessary overhead since Task is a reference type requiring memory allocation.

For instance, consider retrieving a list of countries as master data (GetCountries) from the database, and due to the infrequency of changes, you decide to cache this data in memory. GetCountries involves an I/O operation when retrieved from the database, and to prevent thread blocking, the method is made asynchronous, returning a Task<Dictionary<int, string>> object. After the initial call, subsequent calls to GetCountries do not necessitate Task allocation on the heap since the data is already available in memory.

ValueTask is a value type. ValueTask<T> can be awaited similar to a Task<T>. ValueTask<T> can assist in this scenario by either synchronously returning T, the value available in memory (in this case, the list of countries), or a Task<T>, an incomplete Task that may complete in the future. See the example below for clarification.

However, it's crucial to assess the need for using ValueTask<T> before implementing it to determine if such optimization is genuinely required. There exist a few limitations of ValueTask<T>:

- After the await, the ValueTask instance is marked as completed, attempting to await it again will result in an exception or unexpected behavior.
- The GetAwaiter() method, typically used internally by the await keyword to await a task or task-like object, when used prematurely on a ValueTask, can lead to unpredictable behavior or exceptions.
- Conversion of a ValueTask to a Task using the AsTask() method is possible, but it can be called only once.