Consider interface as a definition for the outside users, what does functionality in an object look like if I'd call it. Interface describes the 'signature' of methods and properties as others see it.
From
Access Modifiers (C# Programming Guide)[
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"Interfaces declared directly within a namespace can be declared as public or internal and, just like classes and structs, interfaces default to internal access. Interface members are always public because the purpose of an interface is to enable other types to access a class or struct. No access modifiers can be applied to interface members."
Now what interface doesn't define is that it doesn't define how, using what methods, a functionality is built. So it's not a definition, how to implement but how to be used from outside once implemented.
Having that said, it wouldn't make sense to define constraints, what must be implemented inside an interface since that's not it's purpose
An example:
Think about a digital thermometer. It shows you the temperature, it may have a functionality to remember the highest and the lowest values, those values may be reset using a button and perhaps you can change the scale between Fahrenheit and Celsius. This is you interface, how to use it.
What you don't see is how the thermometer is built internally what components are used, how is the wiring done and so on. More importantly those do not matter since if the manufacturer makes the same model year after year, they may find more applicable components and the internal design may change. Perhaps some wiring is taken away and replaced by more advanced chip etc.
All these changes in inner parts have no effect on how to use the thermometer. The interface stays the same even if the internal implementation may change dramatically.
From the user point of view, you're not interested how the thermometer is built, only how you use it. Because of that the inner logic may not be defined in the definition of the interface :)
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