This measures a single qubit in the Pauli-Z basis such that the output result is given by the distribution. The M operation is defined as:

Pr(Zero||ψ
 ⟩
 )=
 ⟨
 ψ|0
 ⟩⟨
 0|ψ
 ⟩

How does the Q# language differ from C#? Here are a few points.

The using statement is different from that of C#. It's used to allocate an array of qubits for processing. Like the using statement in C#, the qubits are released at the end of the using statement. There are no qubits that are used for the entire lifetime of the application.

Q# has a different for loop that is used to iterate through a range. There is no direct equivalent of a C# for loop.

Q#, by default, has all variables as immutable, meaning that once they are assigned, they cannot be changed. There is a let keyword that will allow binding of the variable. Operational arguments are always immutable regardless. That being said, there is (at the time of writing) the ability to declare a variable with the mutable keyword and set the value later using the set statement.