Open Visual Studio, go to View > Object Browser , and add the DLL to see its classes and methods.
Tools like .NET Reflector or ILSpy can reconstruct the original source code from the DLL. Common Use Cases Stopwatch Class (System.Diagnostics) - Microsoft Learn stopwatch.dll
If you have a file named stopwatch.dll and need to see its specific contents or "API," you can use the following tools: Open Visual Studio, go to View > Object