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.NET Forum / .NET Framework / New Users / August 2005ToString.....or not ToString (that is the question)
Although the object class has a ToString method, not all classes show this method in their IntelliSense list of properties and methods. Why? Also, in VB .NET, with Option Strict turned on, you can sometimes get away with assigning a non-string type to a string, without having to perform a cast or ToString on the value being used. Why? > Although the object class has a ToString method, not all classes show this > method in their IntelliSense list of properties and methods. Why? > > Also, in VB .NET, with Option Strict turned on, you can sometimes get away > with assigning a non-string type to a string, without having to perform a > cast or ToString on the value being used. Why? Please provide a specific example where implicit conversion succeeded with Option Strict on.  SignaturePeter [MVP Visual Developer] Jack of all trades, master of none. One implicit conversion that compiles with Option Strict On is Char to String. e.g., Dim thisString As String = "a"c (the same isn't true in C# - char's never implicitly convert to strings) I was sure there was another case or two, but I can't recall right now. Option Strict On is stricter than C# in some ways, but not as strict as C# in other ways. SignatureDavid Anton www.tangiblesoftwaresolutions.com Home of: Clear VB: Cleans up outdated VB.NET code Instant C#: Converts from VB.NET to C# Instant VB: Converts from C# to VB.NET Instant J#: Converts from VB.NET to J# > > Although the object class has a ToString method, not all classes show this > > method in their IntelliSense list of properties and methods. Why? [quoted text clipped - 5 lines] > Please provide a specific example where implicit conversion succeeded with > Option Strict on. > One implicit conversion that compiles with Option Strict On is Char to String. > e.g., [quoted text clipped - 5 lines] > Option Strict On is stricter than C# in some ways, but not as strict as C# > in other ways. A char can be converted to a string w/o losing any precision. Just like you can convert an integer to a double w/o the possibility of losing any data the same holds true for a char to a string. It will allow you do the the implicit conversion anytime you don't have the chance of losing any data. Chris Exactly - this was just an example of an implicit conversion that is allowed in VB with Option Strict On (the second poster asked for an example). It is strange that it wouldn't be allowed in C# however (while C# does allow other implicit conversions though). SignatureDavid Anton www.tangiblesoftwaresolutions.com Home of: Clear VB: Cleans up outdated VB.NET code Instant C#: Converts from VB.NET to C# Instant VB: Converts from C# to VB.NET Instant J#: Converts from VB.NET to J# > > One implicit conversion that compiles with Option Strict On is Char to String. > > e.g., [quoted text clipped - 13 lines] > > Chris See my other replies. This example has taken us away from the original questions. > Exactly - this was just an example of an implicit conversion that is > allowed [quoted text clipped - 24 lines] >> >> Chris Not true. As stated in my first reply, what Dave showed is not an example of implicit conversion. The "c" character is tells the CLR what data type to use on the string in the first place (in this case: char). Because of that, the issue of casting becomes irrelevant because "a" is assigned as a char in the first place. >> One implicit conversion that compiles with Option Strict On is Char to >> String. [quoted text clipped - 14 lines] > > Chris That's not an implicit conversion. That's using a type literal to specify the type to use in the first place, so that no conversion (cast) is needed later. > One implicit conversion that compiles with Option Strict On is Char to > String. [quoted text clipped - 20 lines] >> with >> Option Strict on. Not quite - "a"c is a character, not a string. So to assign this to a string without explicit casting is indeed an implicit cast. The fact that there's no doubt about the type of the right hand side of an assignment has nothing to do with whether an implicit cast is taking place. e.g., Dim c As Char = "a"c 'no casting Dim s As String = "a" 'no casting Dim s As String = CStr("a"c) 'explicit cast Dim s As String = "a"c 'implicit cast SignatureDavid Anton www.tangiblesoftwaresolutions.com Home of: Clear VB: Cleans up outdated VB.NET code Instant C#: Converts from VB.NET to C# Instant VB: Converts from C# to VB.NET Instant J#: Converts from VB.NET to J# > That's not an implicit conversion. That's using a type literal to specify > the type to use in the first place, so that no conversion (cast) is needed [quoted text clipped - 24 lines] > >> with > >> Option Strict on. You are missing the point. I'm not referring to the left hand side of the expression. I was talking about the type literal not being a cast. In any case, this is not a scenario that I am asking about (strings and chars) in the first place. > Not quite - "a"c is a character, not a string. So to assign this to a > string [quoted text clipped - 44 lines] >> >> with >> >> Option Strict on. To follow up, whether or not precision is lost is not a factor when considering if Option Strict is going to consider something an explicit or implicit cast. An Integer can be cast as a long without losing precision, but with Option Strict on, you still need to explicitly cast that Integer as a Long if you wish to use it as a long. > Not quite - "a"c is a character, not a string. So to assign this to a > string [quoted text clipped - 44 lines] >> >> with >> >> Option Strict on. Ok, I just tried the Integer and Long scenario and, I'll admit, I got that one wrong. What I'm really trying to understand is why a DataSet would allow a string value passed into a Row Item (which is of the object type) with Option Strict On, but other data types wouldn't work and why ToString does not appear on the member list of every class, since ToString is a member of the Object class. > To follow up, whether or not precision is lost is not a factor when > considering if Option Strict is going to consider something an explicit or [quoted text clipped - 53 lines] >>> >> with >>> >> Option Strict on. > Please provide a specific example where implicit conversion succeeded with > Option Strict on. In a Console Application, you can do this (with Option Strict On): Dim x As Integer = 10 Console.WriteLine("The value is" & x) ...here's another example (with Option Strict On) that works: Assume we have a loosely-typed DataSet (ds) with a DataTable that has a Column (RetailPrice) that is set to hold System.Double data: ds.Tables(0).Rows(0).Item("RetailPrice") = txtPrice.Text NOTE: While you may look at this and say that at design-time, no error is seen because the DataSet doesn't know until run-time that the data is bad. Fair enough, but when I run this code it does not throw any exceptions. The text data is converted to Double data and placed in the DataSet. ALSO: No one has addressed my first question of why, although Objects have a ToString method, not ALL classes have it, however you can still use ToString (with Option Strict turned on) on classes that don't show it in their IntelliSense list of members. Thanks. Inline.. ---- Nigel Norris >> Please provide a specific example where implicit conversion succeeded >> with Option Strict on. [quoted text clipped - 14 lines] > Fair enough, but when I run this code it does not throw any exceptions. > The text data is converted to Double data and placed in the DataSet. This is nothing to do with casting - the assignment is object = string, so no cast is required. The conversion that is going on is an ADO.NET feature. When storing your assigned value (an object) to a column of type double it calls Convert.ToDouble(yourvalue). I'm not sure where (or even if) this is documented, but that's what it does. > ALSO: No one has addressed my first question of why, although Objects > have a ToString method, not ALL classes have it, however you can still use > ToString (with Option Strict turned on) on classes that don't show it in > their IntelliSense list of members. Can you provide a specific example? IntelliSense always seems to list ToString on any class I give it (as it should). Inline: > This is nothing to do with casting - the assignment is object = string, so > no cast is required. The conversion that is going on is an ADO.NET > feature. When storing your assigned value (an object) to a column of type > double it calls Convert.ToDouble(yourvalue). I'm not sure where (or even > if) this is documented, but that's what it does. That sounds a lot like an implicit cast, which shouldn't happen with Option Strict turned on (hence my question). Also, why doesn't is do something similar if I try to pass a short (rather than a string) in the same circumstance? >> ALSO: No one has addressed my first question of why, although Objects >> have a ToString method, not ALL classes have it, however you can still [quoted text clipped - 3 lines] > Can you provide a specific example? IntelliSense always seems to list > ToString on any class I give it (as it should). Sure: Dim sb As New StringBuilder sb.Append("<Product>" & CType(dr("Name"), String) & "</Product>") If you type dr("Name") and then a dot (.), you will NOT see ToString listed (although you can still type it and it will work). This scenario also happens in several other situations as well. [snip] > Sure: > Dim sb As New StringBuilder [quoted text clipped - 3 lines] > (although you can still type it and it will work). This scenario also > happens in several other situations as well. There is an option within the Text Editor/Basic options to disable listing of "Advanced Members". If you turn that off, you should then see the ToString method (assuming dr is a DataReader - you didn't give us the right Dim). What I cannot seem to find is how to define your own members as advanced - does anyone know of the attribute to use? Damien Inline > [snip] >> [quoted text clipped - 9 lines] > There is an option within the Text Editor/Basic options to disable > listing of "Advanced Members". You're quite right, though it seems to me a slightly strange choice to make that particular method "Advanced" - doesn't seem to quite fit the bill to me. >If you turn that off, you should then > see the ToString method (assuming dr is a DataReader - you didn't give > us the right Dim). What I cannot seem to find is how to define your own > members as advanced - does anyone know of the attribute to use? > > Damien I believe the attribute is System.ComponentModel.EditBrowsableAttribute with a state of Advanced. Regards, Nick Hall > Inline > > [quoted text clipped - 15 lines] > that particular method "Advanced" - doesn't seem to quite fit the bill to > me. I think the "Advanced" is more of a "don't clutter up Intellisense for things that they're unlikely to want to do" setting - and in this case, I think that there are other more likely things to want to do with an object. Whereas with, for instance, an Integer, it's a fairly common thing with Integers to want to display them in text output, so it's left as normal. > >If you turn that off, you should then > > see the ToString method (assuming dr is a DataReader - you didn't give [quoted text clipped - 5 lines] > I believe the attribute is System.ComponentModel.EditBrowsableAttribute with > a state of Advanced. Thanks Nick. Not sure I've got any use for it yet, but it always helps in the future when someone asks me to do something and it tickles back to the back of the brain :-) Damien Thanks Damien. I agree that this doesn't seem like it would be an advanced member (especially since even with that turned on you do see ToString in many other situations). > [snip] >> [quoted text clipped - 14 lines] > > Damien >> This is nothing to do with casting - the assignment is object = string, >> so no cast is required. The conversion that is going on is an ADO.NET [quoted text clipped - 4 lines] > That sounds a lot like an implicit cast, which shouldn't happen with > Option Strict turned on (hence my question). OK - we have to start getting precise about terminology. Since I'm a C# user, what I say may need some VB adjustment. A cast is a request to perform a conversion. There are implicit conversions and explicit conversions. Implicit conversions are all information preserving. Explicit conversions in general may lose information or fail. The compiler will perform implicit conversions automatically. In C# you need to use a cast to request an explicit conversion. In VB, as I understand it, if Option Strict is turned off, the compiler will provide explicit conversions automatically as well. With Option Strict turned on, it behaves like C#. An assignment of the form object = string is an implicit conversion (all reference types can be converted to object). Or, in another form, object is a supertype of string and there is always an implict conversion to a supertype. So, you can assign a string value to a double column in a row quite happily as far as the compiler is concerned, irrespective of the option setting. At runtime ADO.NET calls the conversion on the passed in (string) object. > Also, why doesn't is do something similar if I try to pass a short (rather > than a string) in the same circumstance? It does. Assigning a short to an object variable is an example of a boxing conversion, which is another implicit conversion. So setting the column value passes in a boxed short value. ADO.NET will then apply the appropriate conversion to a double. This works OK for me (in VB!).
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