The difference between First,firstordefault,single,singleordefault

Operator	If the source sequence is empty	The source sequence consists of only one element	The source sequence contains multiple elements
First	Throw exception	Returns the element	Returns the first element
FirstOrDefault	Return to Default (TSource)	Returns the element	Returns the first element
Last	Throw exception	Returns the element	Returns the last element
LastOrDefault	Return to Default (TSource)	Returns the element	Returns the last element
Single	Throw exception	Returns the element	Throw exception
Singleordefault	Return to Default (TSource)	Returns the element	Throw exception

Obviously, if the input sequence has only one element, the execution results of these operators are very consistent:) Similarly, if the input sequence is empty, then the operator without "Ordefault" throws an exception (InvalidOperationException) with "Ordefault" operator returns the default value of the element type (the default value for the reference type is Null,int, 0, and so on).

If the input sequence (possibly filtered) contains multiple elements, the results of the execution of these operators differ greatly, and the result of first and last is the name, and single throws an exception. It is important to note that Singleordefault also throws an exception because it does not look like this: if the input sequence has only one element, the element is returned, otherwise the default value is returned. If you need an operator that can handle a multi-element sequence, use first or last. If you need to deal with sequences that may be empty, use FirstOrDefault or LastOrDefault. Note that if you use an operator with "Ordefault", then an empty sequence and a sequence with only the default values will perform exactly the same.

The difference between First,firstordefault,single,singleordefault