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Why doesn't a String of Nothing cause cause an exception? (BC42104) [duplicate]

Difference between Equals/equals and == operator?Nothing = String.Empty (Why are these equal?)Why is Dictionary preferred over Hashtable in C#?Compiler warning: null reference exceptionVariable 'cl' is passed by reference before it has been assigned a value for DataView Variablewarning BC42104 - Variable used before assigned a valueNull Reference List(of String)Why does checking if an object is null cause a warning at compile time?Visual Basic warningvariable “stat_in” is used before it has been assigned a value. a null exception could result at runtimeVariable 'reader' is used before it has been assigned a value. A null reference exception could result at run timeVB.Net no idea how to get this to compile also Retro BASIC

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;

0

This question already has an answer here:

• 
  Difference between Equals/equals and == operator?
  
  11 answers
  
  

If I declare a string but do not assign a value, the Equals function throws an exception but it doesn't throw an exception if it is compared to a value.

The error list warns about the problem:

Warning BC42104 Variable a is used before it has been assigned a
value. A null reference exception could result at runtime.

Dim a As String
Dim b as string = "bar"

a.Equals("foo") 'causes System.NullReferenceException

a = "foo" 'No exception although a is nothing

a = b 'No exception although a is nothing

I know the warning says it COULD cause an exception, but does anyone know why this is the happening?

vb.net

share|improve this question

edited Mar 27 at 19:13

Milo

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asked Mar 27 at 18:51

UggyUggy

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marked as duplicate by Cody Gray♦ vb.net
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Mar 27 at 19:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Nothing and Strings in Visual Basic
  
  – TnTinMn
  Mar 27 at 19:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The warning is pretty much unrelated. It can always be Nothing, even if initialized; for example, the warning will go away if you write Dim a As String = Nothing. Is your real question about the difference between the = operator and the .Equals method?
  
  – Ry-♦
  Mar 27 at 19:15
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  a.Equals is calling a member function on a. Obviously you cannot call a member function on an object that is "null"/Nothing. However, the equality operator is special-cased to handle null objects.
  
  – Cody Gray♦
  Mar 27 at 19:16
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  (Also, please confirm that you’re using a = "foo" and a = b in an expression and not as a statement – preferably with real code – because it’s also a valid assignment statement. Since you already got an answer that was confused about that.)
  
  – Ry-♦
  Mar 27 at 19:16
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  To elaborate on what @CodyGray is saying, also note that string.operator==(string, string), like all overloaded operators, is static (Shared), at no point does the .NET framework try to do a.Equals when it handles the = operator; string.Equals(string), on the other hand is an instance method of String and will cause a NullReferenceException.
  
  – jrh
  Mar 27 at 19:20
  
  

  
  
  
  

 | 
show 8 more comments

0

This question already has an answer here:

• 
  Difference between Equals/equals and == operator?
  
  11 answers
  
  

If I declare a string but do not assign a value, the Equals function throws an exception but it doesn't throw an exception if it is compared to a value.

The error list warns about the problem:

Warning BC42104 Variable a is used before it has been assigned a
value. A null reference exception could result at runtime.

Dim a As String
Dim b as string = "bar"

a.Equals("foo") 'causes System.NullReferenceException

a = "foo" 'No exception although a is nothing

a = b 'No exception although a is nothing

I know the warning says it COULD cause an exception, but does anyone know why this is the happening?

vb.net

share|improve this question

edited Mar 27 at 19:13

Milo

2,11166 gold badges1919 silver badges3333 bronze badges

asked Mar 27 at 18:51

UggyUggy

322 bronze badges

marked as duplicate by Cody Gray♦ vb.net
Users with the  vb.net badge can single-handedly close vb.net questions as duplicates and reopen them as needed.

StackExchange.ready(function()
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$('.dupe-hammer-message-hover:not(.hover-bound)').each(function()
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$hover.hover(
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Mar 27 at 19:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Nothing and Strings in Visual Basic
  
  – TnTinMn
  Mar 27 at 19:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The warning is pretty much unrelated. It can always be Nothing, even if initialized; for example, the warning will go away if you write Dim a As String = Nothing. Is your real question about the difference between the = operator and the .Equals method?
  
  – Ry-♦
  Mar 27 at 19:15
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  a.Equals is calling a member function on a. Obviously you cannot call a member function on an object that is "null"/Nothing. However, the equality operator is special-cased to handle null objects.
  
  – Cody Gray♦
  Mar 27 at 19:16
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  (Also, please confirm that you’re using a = "foo" and a = b in an expression and not as a statement – preferably with real code – because it’s also a valid assignment statement. Since you already got an answer that was confused about that.)
  
  – Ry-♦
  Mar 27 at 19:16
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  To elaborate on what @CodyGray is saying, also note that string.operator==(string, string), like all overloaded operators, is static (Shared), at no point does the .NET framework try to do a.Equals when it handles the = operator; string.Equals(string), on the other hand is an instance method of String and will cause a NullReferenceException.
  
  – jrh
  Mar 27 at 19:20
  
  

  
  
  
  

 | 
show 8 more comments

This question already has an answer here:

• 
  Difference between Equals/equals and == operator?
  
  11 answers
  
  

If I declare a string but do not assign a value, the Equals function throws an exception but it doesn't throw an exception if it is compared to a value.

The error list warns about the problem:

Warning BC42104 Variable a is used before it has been assigned a
value. A null reference exception could result at runtime.

Dim a As String
Dim b as string = "bar"

a.Equals("foo") 'causes System.NullReferenceException

a = "foo" 'No exception although a is nothing

a = b 'No exception although a is nothing

I know the warning says it COULD cause an exception, but does anyone know why this is the happening?

vb.net

share|improve this question

edited Mar 27 at 19:13

Milo

2,11166 gold badges1919 silver badges3333 bronze badges

asked Mar 27 at 18:51

UggyUggy

322 bronze badges

Dim a As String
Dim b as string = "bar"

a.Equals("foo") 'causes System.NullReferenceException

a = "foo" 'No exception although a is nothing

a = b 'No exception although a is nothing

share|improve this question

edited Mar 27 at 19:13

Milo

2,11166 gold badges1919 silver badges3333 bronze badges

asked Mar 27 at 18:51

UggyUggy

322 bronze badges

share|improve this question

edited Mar 27 at 19:13

Milo

2,11166 gold badges1919 silver badges3333 bronze badges

asked Mar 27 at 18:51

UggyUggy

322 bronze badges

edited Mar 27 at 19:13

Milo

2,11166 gold badges1919 silver badges3333 bronze badges

edited Mar 27 at 19:13

Milo

2,11166 gold badges1919 silver badges3333 bronze badges

asked Mar 27 at 18:51

UggyUggy

322 bronze badges

asked Mar 27 at 18:51

UggyUggy

322 bronze badges

1 Answer
1

active

oldest

votes

1

This is because Dim a As String declares the type of variable a but doesn't assign anything to it. This is basically saying : "This variable was made to hold a String object, but it doesn't hold any right now". On the other hand, Dim b As String = "bar" declares the variable and its type, but also assigns a String object to it ("bar"). The reason a.Equals("foo") returns an exception is because you only declared it without assigning anything to it (so you are trying to access an object that isn't there). a = "foo" works because you are assigning a String object of value "foo" to the variable a. It's like saying : "This variable now holds a String object with value 'foo'".

Edit :

While your code points to the assignment of the variable a, I was made aware that you wanted to know why the = operator, as a comparison operator, works. This is because what I said earlier isn't entirely true when I said it didn't hold an object. It actually is of Nothing value (which sets it as a null reference) if no String object was assigned to it (String is a nullable object).

See : https://docs.microsoft.com/en-us/dotnet/visual-basic/language-reference/nothing

Hope this helps.

share|improve this answer

edited Mar 27 at 19:32

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The question is about string comparison -- "it doesn't throw an exception if it is compared to a value". The code presented leads one to assume it is about assignment.
  
  – TnTinMn
  Mar 27 at 19:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TnTinMn this is true. I was looking at the code given. If the a = "foo" expression is used in an If statement, I assume it works because a is treated as a Nothing object, which works with the = operator.
  
  – Olivier Samson
  Mar 27 at 19:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You might as well cover both contingencies in your answer before someone else does. Also look at the link I posted as comment.
  
  – TnTinMn
  Mar 27 at 19:30
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes this was in an If statement. Hence I was expecting an exception (like in C#) but reading TnTinMn link I see now that the VB runtime treats a string of nothing as an empty string, which now makes sense of this.
  
  – Uggy
  Mar 28 at 8:42
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Uggy: ”Like in C#” – ? null == "foo" works fine in C# too with no exception.
  
  – Ry-♦
  Mar 28 at 20:35
  
  

  
  
  
  

add a comment | 

1

This is because Dim a As String declares the type of variable a but doesn't assign anything to it. This is basically saying : "This variable was made to hold a String object, but it doesn't hold any right now". On the other hand, Dim b As String = "bar" declares the variable and its type, but also assigns a String object to it ("bar"). The reason a.Equals("foo") returns an exception is because you only declared it without assigning anything to it (so you are trying to access an object that isn't there). a = "foo" works because you are assigning a String object of value "foo" to the variable a. It's like saying : "This variable now holds a String object with value 'foo'".

Edit :

While your code points to the assignment of the variable a, I was made aware that you wanted to know why the = operator, as a comparison operator, works. This is because what I said earlier isn't entirely true when I said it didn't hold an object. It actually is of Nothing value (which sets it as a null reference) if no String object was assigned to it (String is a nullable object).

See : https://docs.microsoft.com/en-us/dotnet/visual-basic/language-reference/nothing

Hope this helps.

share|improve this answer

edited Mar 27 at 19:32

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The question is about string comparison -- "it doesn't throw an exception if it is compared to a value". The code presented leads one to assume it is about assignment.
  
  – TnTinMn
  Mar 27 at 19:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TnTinMn this is true. I was looking at the code given. If the a = "foo" expression is used in an If statement, I assume it works because a is treated as a Nothing object, which works with the = operator.
  
  – Olivier Samson
  Mar 27 at 19:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You might as well cover both contingencies in your answer before someone else does. Also look at the link I posted as comment.
  
  – TnTinMn
  Mar 27 at 19:30
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes this was in an If statement. Hence I was expecting an exception (like in C#) but reading TnTinMn link I see now that the VB runtime treats a string of nothing as an empty string, which now makes sense of this.
  
  – Uggy
  Mar 28 at 8:42
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Uggy: ”Like in C#” – ? null == "foo" works fine in C# too with no exception.
  
  – Ry-♦
  Mar 28 at 20:35
  
  

  
  
  
  

add a comment | 

1

This is because Dim a As String declares the type of variable a but doesn't assign anything to it. This is basically saying : "This variable was made to hold a String object, but it doesn't hold any right now". On the other hand, Dim b As String = "bar" declares the variable and its type, but also assigns a String object to it ("bar"). The reason a.Equals("foo") returns an exception is because you only declared it without assigning anything to it (so you are trying to access an object that isn't there). a = "foo" works because you are assigning a String object of value "foo" to the variable a. It's like saying : "This variable now holds a String object with value 'foo'".

Edit :

While your code points to the assignment of the variable a, I was made aware that you wanted to know why the = operator, as a comparison operator, works. This is because what I said earlier isn't entirely true when I said it didn't hold an object. It actually is of Nothing value (which sets it as a null reference) if no String object was assigned to it (String is a nullable object).

See : https://docs.microsoft.com/en-us/dotnet/visual-basic/language-reference/nothing

Hope this helps.

share|improve this answer

edited Mar 27 at 19:32

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The question is about string comparison -- "it doesn't throw an exception if it is compared to a value". The code presented leads one to assume it is about assignment.
  
  – TnTinMn
  Mar 27 at 19:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TnTinMn this is true. I was looking at the code given. If the a = "foo" expression is used in an If statement, I assume it works because a is treated as a Nothing object, which works with the = operator.
  
  – Olivier Samson
  Mar 27 at 19:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You might as well cover both contingencies in your answer before someone else does. Also look at the link I posted as comment.
  
  – TnTinMn
  Mar 27 at 19:30
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes this was in an If statement. Hence I was expecting an exception (like in C#) but reading TnTinMn link I see now that the VB runtime treats a string of nothing as an empty string, which now makes sense of this.
  
  – Uggy
  Mar 28 at 8:42
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Uggy: ”Like in C#” – ? null == "foo" works fine in C# too with no exception.
  
  – Ry-♦
  Mar 28 at 20:35
  
  

  
  
  
  

add a comment | 

This is because Dim a As String declares the type of variable a but doesn't assign anything to it. This is basically saying : "This variable was made to hold a String object, but it doesn't hold any right now". On the other hand, Dim b As String = "bar" declares the variable and its type, but also assigns a String object to it ("bar"). The reason a.Equals("foo") returns an exception is because you only declared it without assigning anything to it (so you are trying to access an object that isn't there). a = "foo" works because you are assigning a String object of value "foo" to the variable a. It's like saying : "This variable now holds a String object with value 'foo'".

Edit :

While your code points to the assignment of the variable a, I was made aware that you wanted to know why the = operator, as a comparison operator, works. This is because what I said earlier isn't entirely true when I said it didn't hold an object. It actually is of Nothing value (which sets it as a null reference) if no String object was assigned to it (String is a nullable object).

See : https://docs.microsoft.com/en-us/dotnet/visual-basic/language-reference/nothing

Hope this helps.

share|improve this answer

edited Mar 27 at 19:32

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges

share|improve this answer

edited Mar 27 at 19:32

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges

answered Mar 27 at 19:09

Olivier SamsonOlivier Samson

51122 silver badges1111 bronze badges