Null(In C#) Vs Nothing(in vb.net)

Question

How is C# NULL different from vb.net Nothing?

Console.WriteLine(Nothing = "") => True

vs

Console.WriteLine(null==""); => False

My understanding was that both null and Nothing are same. But above code clearly explains it is not.

What is the equivalent of C# null in VB.NET?

Answer 1

You have chosen little bid complicated example to test difference between Nothing in vb.net and null in c#

From Visual basic language specification about Nothing:

Nothing is a special literal; it does not have a type and is convertible to all types in the type system, including type parameters. When converted to a particular type, it is the equivalent of the default value of that type.

If you read description of default value expressions from Microsoft documentation you will notice that Nothing in vb.net and default(T) in C# have similar behaviour.

For example

Dim isSomething As Boolean = Nothing ' = False
Dim amount As Integer = Nothing ' = 0
Dim value As String = Nothing ' = null (notice not empty string!)

As prove that default value for string is not empty string, as many of comments/answers here stays

Dim value As String = Nothing
If value.Equals("") Then ' will throw NullReferenceException

End If 

Comparing Nothing to empty string "" with "=" operator is special case for vb.net, because Nothing for string will evaluate to null

From Visual basic language specification about relational operators for type String:

The operators return the result of comparing the two values using either a binary comparison or a text comparison. The comparison used is determined by the compilation environment and the Option Compare statement. A binary comparison determines whether the numeric Unicode value of each character in each string is the same. A text comparison does a Unicode text comparison based on the current culture in use on the .NET Framework. When doing a string comparison, a null reference is equivalent to the string literal "".

For checking string equality in vb.net you should use String.Equals method or Is operator.

Based on above language specifications

My understanding was that both null and Nothing are same. But above code clearly explains it is not.

They are not same, Nothing in vb.net is equivalent to default(T) in C#

What is the equivalent of C# null in VB.NET?

In C# you cannot set null to value types, so for reference types (String) equivalent for C# null in vb.net will be Nothing.
For example (null == default(String))

Answer 2

The final version of the accepted answer correctly answers the question. However, Fabio added some useful information to his answer, but it has a number of incorrect information and concepts. So this answer is an attempt to merge the two answers in a more complete one with the correct information.

First of all, Nothing in VB is equivalent to null in C#, so your understanding of that is correct, at least partially (because that's only one use of the Nothing keyword). It's been reflected throughout the MSDN literature, although it's been fading recently and being replaced with the word null in many comments. Your confusion arises from the behavior and features of VB which differ from C# and programmers who are coming from other languages and not experienced in VB get confused by the results.

As stated in the accepted answer, the question is considered a string comparison by VB because one of the operands is a string. In string comparisons, VB treats Nothing the same as the empty string "" as per MSDN:

Numeric comparisons treat Nothing as 0. String comparisons treat Nothing as "" (an empty string).

Now, why is that happening in VB? It happens because VB uses the Nothing keyword for two different things (similar to how it uses the = operator for two different things, assignment and comparison). The first usage is equivalent to null in C# as I mentioned above. The second usage is equivalent to the default(T) operator in C#. Like with the = operator, VB knows which way to use Nothing depending on the context. Examples:

In this line, VB uses Nothing as null:

Console.WriteLine(s Is Nothing) //C#: (s == null)

In these lines, VB uses Nothing as default(T) to assign the default value to the variable:

Dim s As String = Nothing //C#: string s = default(string);
Dim i As Integer = Nothing //C#: int i = default(int);

Finally, when comparing variables of different types, both VB and C# convert one operand to match the type of the other operand. For example, if you compare double and integer values. Both VB and C# will return true for this line:

Console.WriteLine(5.0 = 5) //C#: (5.0 == 5)

However, VB and C# differ in what types can be converted to what. Also, the empty string "" and null are treated the same in string comparisons in VB, but not in C#. This makes things easier in most cases, which is something known about VB in general. So you can easily test if your string has no value like this:

Console.WriteLine(s = Nothing)
Console.WriteLine(s = "")

Both lines will return the same result because VB treats Nothing as "" for comparison. In C#, you cannot do that and you have to test for both like this:

Console.WriteLine(s == null && s == "")

Obviously, VB is easier and shorter than C# in this case which is the most common one. However, this comes at the cost of losing some control. In the less common cases when you only want to test for null reference, C# become obviously better because you can still use the == operator:

Console.WriteLine(s == null)

While in VB, you cannot use the = operator and you have to use another one, the Is operator:

Console.WriteLine(s Is Nothing)

Also although VB is easier in the previous point, C# is clearer. That's why from code clarity perspective, this line can be used instead in both languages with the same result:

Console.WriteLine(string.IsNullOrEmpty(s))

Answer 3

In your code, VB guesses that you are comparing Strings, since one of the operands is a String. In String comparisons, Nothing is equivalent to the empty String "". It then does a value comparison, which returns True.

Use Is to compare references:

Console.WriteLine(Nothing Is "") '=> False

Answer 4

Nothing in VB varies by the objects type: - Reference types equate to null - Value types equate to the default value; which for a nullable type is null