public AndConstraint<TAssertions> Contain( 
   string expected,
   OccurrenceConstraint occurrenceConstraint,
   string because,
   params object[] becauseArgs
)

Parameters

expected

The (fragment of a) string that the current string should contain.

occurrenceConstraint

A constraint specifying the amount of times a substring should be present within the test subject. It can be created by invoking static methods Once, Twice, Thrice, or Times(int) on the classes FluentAssertions.Exactly, FluentAssertions.AtLeast, FluentAssertions.MoreThan, FluentAssertions.AtMost, and FluentAssertions.LessThan. For example, FluentAssertions.Exactly.Times or FluentAssertions.LessThan.Twice.

because

A formatted phrase as is supported by System.String.Format(System.String,System.Object[]) explaining why the assertion is needed. If the phrase does not start with the word because, it is prepended automatically.

becauseArgs

Zero or more objects to format using the placeholders in because.

Reference

StringAssertions<TAssertions> Class
StringAssertions<TAssertions> Members
Overload List
Exactly Class
AtLeast Class
MoreThan Class
AtMost Class
LessThan Class
Times Method
Twice Method
System.String.Format(System.String,System.Object[])