First, Single, and their OrDefault variants in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the First() method do in C#?

It returns the first element of a sequence. If the sequence is empty, it throws an exception.

beginner

How does FirstOrDefault() differ from First()?

FirstOrDefault() returns the first element if found; otherwise, it returns the default value for the type (e.g., null for reference types, 0 for integers) instead of throwing an exception.

intermediate

What is the key difference between Single() and First()?

Single() expects exactly one element in the sequence and throws an exception if there are zero or more than one elements. First() only requires at least one element and returns the first one.

intermediate

When should you use SingleOrDefault()?

Use SingleOrDefault() when you expect zero or one element in the sequence. It returns the single element if found, or the default value if none are found. It throws an exception if more than one element exists.

beginner

What happens if you call Single() on a sequence with multiple elements?

It throws an InvalidOperationException because Single() expects exactly one element, not multiple.

What does FirstOrDefault() return if the sequence is empty?

A Returns the default value for the type

B Returns the first element

C Returns null always

D Throws an exception

FirstOrDefault() returns the default value for the type if the sequence is empty, such as null for reference types or 0 for integers.

Which method throws an exception if the sequence contains more than one element?

A Single()

B First()

C FirstOrDefault()

D SingleOrDefault()

Single() throws an exception if there is more than one element in the sequence.

If you want to get the first element but avoid exceptions on empty sequences, which method should you use?

A Single()

B FirstOrDefault()

C First()

D SingleOrDefault()

FirstOrDefault() safely returns the first element or a default value if the sequence is empty.

What will SingleOrDefault() return if the sequence has exactly one element?

A Throws an exception

B Returns default value

C Returns the single element

D Returns the first element

SingleOrDefault() returns the single element if exactly one exists.

Which method is best when you expect exactly one element and want an exception if zero or multiple elements exist?

A SingleOrDefault()

B FirstOrDefault()

C First()

D Single()

Single() enforces exactly one element and throws if zero or more than one exist.

Explain the difference between First() and Single() methods in C#.

Think about how many elements each method expects and what happens if the sequence is empty or has many elements.

When would you choose to use FirstOrDefault() or SingleOrDefault()?

Consider the expected number of elements and how you want to handle empty sequences.