ValueType object. Classes, on the other hand, are reference type variables. Instances of classes are normally allocated on heap memory. Classes support inheritance. Even though user-defined types such as classes and structures are supported in Visual Basic. NET, fixed-length strings in user-defined types are not supported.
In such cases, data marshalling techniques must be used in Visual Basic. Variants are initialized to Empty , which automatically gets converted to zero when used in a numeric expression, or to an empty string when used in a string expression. Empty is not supported in Visual Basic. Object type variables are initialized to Nothing in Visual Basic. Null values indicate that a variable contains no valid data. If any part of an expression evaluates to null , the entire expression evaluates to a null.
Null propagation is not supported in Visual Basic. Normally null values are obtained when database operations are involved. Because null propagation is not supported, Visual Basic. NET offers a database-programming model, which test fields explicitly for null before retrieving their values.
The following code snippet shows an example of the DefInt type:. The DefType statements are used at the module level. In the preceding code snippet, all variables that begin with A, B, C, and D are defined to be of type integer. The sample code gives an execution error when the string "Error" is assigned to the variable BVariable, which is of integer type variable. This type of data type definition is not supported in Visual Basic. So the following code is valid in Visual Basic 6.
This type of code is not supported in Visual Basic. If a variable needs to be used outside a block in Visual Basic. NET, it needs to be declared outside of the block. If the preceding code is written in Visual Basic. NET, it will give a compilation error that the variable iLoopCounter has not been declared. There is a very huge difference in the way objects are cleared from memory between Visual Basic 6. When objects are not in cycles, reference counting will immediately detect when an object is no longer being used and will run its termination code.
NET, a garbage collector does the job of cleaning up the unreferenced objects. The garbage collector is a feature of the Microsoft. NET Framework, and it is applicable for all the languages supported by the Microsoft.
NET Framework. The developer does not know when the garbage collector will collect the unreferenced objects. This is known as undeterministic garbage collection. The garbage collector also compacts the memory that is in use. Developers can directly call this method to force garbage collection. However, calling the Collect method too often can cause performance issues, because the garbage collector suspends all executing threads before performing garbage collection.
Hence it is better to let the garbage collector determine when to perform the garbage collection. We can specify arrays with any lower bounds. This is not supported in Visual Basic. All arrays in Visual Basic. NET have to start off with a base of 0 to make the language interoperable with the other languages supported in the Microsoft. In fact, the Option Base statement is not supported in Visual Basic. If the array has not been declared earlier, it is declared with the ReDim statement.
NET, the ReDim statement can be used only if the array has been declared earlier. Normally this statement is placed at the beginning of a block of code, and it handles any errors that occur in that block of code. This is not a structured way of handling errors.
NET has vastly improved the error-handling techniques. NET, conditions in which code fails are known as exceptions. If the block throws an exception, it is caught by the code in the catch block. The catch block can handle the exception on its own, or it can rethrow the exception to a higher exception-handling code. To return a value from a function, in Visual Basic 6. NET, the desired value is returned by the Return statement.
NET, there is only one form of assignment. The value of an object type variable is the reference to the object instances, so if x and y are reference type variables, a reference assignment is performed. NET makes it mandatory to have parentheses in procedure calls.
NET, the Static keyword is not supported in the procedure, and all static local variables need to be explicitly declared with the Static statement. If the developer does not specify it, the parameter passing defaults to ByRef. This has changed to ByVal in Visual Basic.
So the developer has to declare a subroutine or function explicitly as ByRef to get it to change a parameter. NET requires that a default value be specified for all optional parameters. However when variables are passed to a ParamArray argument in Visual Basic. NET, they cannot be modified by the called function. ByRef ParamArray elements are not supported. This was supported to enable calling APIs whose parameters supported two or more data types. NET, overloaded Declare statements have to be defined, and it allows a native API to be called with two or more data types.
The Implements statement in Visual Basic. NET is different than in Visual Basic 6. NET, the Implements keyword can be used only for interfaces. Every method that implements an interface method requires an Implements keyword at the end of the method declaration statement.
This will specify what interface method it implements. For example, the property Get function can be Public , whereas the property Let is Friend. NET, the Get and Set functions for a property must both have the same level of accessibility.
All built-in controls of Visual Basic 6. NET, only those properties that take parameters can be marked as default. It is common for those properties with parameters to be indexers into a collection. NET, enumeration constants can be referenced without qualification only if an Import for the enumeration is added at the file or project level.
However, in Visual Basic. Because these are unstructured ways of coding, they are not supported in Visual Basic. GoSub Return is not a structured way of coding and is not supported in Visual Basic.
NET-world talk about "lazy engineering". We're getting a bit far afield from Variants, but I too don't think anyone was being lazy with respect to the VB6 Boolean type. I certainly agree with Niya that they were maintaining consistency to make sure that And, Or, Not, etc worked the way one would expect especially with backward compatibility concerns.
It is always important to remember that, in VB6, we often use what we might call Boolean operators as Logical operators, but they're not. They're really always Bitwise operators. Not even more typing is required for that I wanna check out what does wReserved2 and wReserved3 do in this structure Olaf lol when I wrote that post I knew I was risking some kind of reaction like this from someone. Well, I'm not trying to start a pissing contest over which language is better.
I think we had quite enough of that in other threads. I was a programming since I was something like 8 years old and for me at that age, things like pointers and memory addressing were difficult for me to grasp so I was unable to learn C or any other more advanced language.
Unified logical and bitwise operators is only one such thing. Other bad habits include things like 1 based array indexing, thinking of strings as first class objects instead of what they really are, a character array, weak typing and many more I can't recall at the moment.
I had no idea how many bad habits I picked up from spending my formative years in it. I didn't and couldn't know any better. All I'm saying, it's not about me trying to start a fight here. It just feels a little more, for lack of a better word, personal to me. Net for all it's perceived flaws does make an attempt to bring the BASIC language more in line with the conventions and norms of other languages which is one of the reasons I like VB.
Net so much. Use C if you want to deal with. It would ingrain better programming habits than VB. If you don't plan on being a serious programmer for example, a hobbyist or maybe you just want to muck about with writing simple scripts, then I highly recommend a BASIC language for it's simplicity. For me, I'm already spoiled. I'm not ready to uproot quite yet but given how fast things are changing with the mobile world, the web, and non-MS platforms actually being viable now, change is inevitable.
I will have to leave MS's world and VB behind eventually. I'm actually seriously considering Java as my new main language in the future because it seems to be the best suited for reaching multiple platforms.
But I'm not quite there yet. Windows still has some life left. Last edited by Niya; Nov 25th, at AM. Hi Baka, For me, it's easy to keep straight if I realize that VB6 boolean operators are made up of a hodge-podge of logical and bitwise operators. However, we must also appreciate that variables declared as Boolean are just a special case of Integers.
And, about the only thing special about them is that the equal sign has that dual purpose function when assigning to them, but not when assigning from another Boolean. Closing notes: 1 Yes, I do wish we had a full compliment of both bitwise and logical operators in VB6 i. If we're dealing with all 1's such as True or all 0's such as False , then the distinction between logical and bitwise operators disappears.
And that's why VB6 uses -1 for True rather than 1. Last edited by Elroy; Nov 25th, at PM. Originally Posted by loquat problem remain not resolved. Loquat, I don't understand. A Variant takes 16 bytes.
This is similar to the way an Integer takes 2 bytes, a Long takes 4 bytes, a Double takes 8 bytes, etc. However, with a Variant, the 16 bytes are used differently depending on a few different things. The first two bytes are always used to indicate the Variant sub-type. As stated above, a Variant can actually be one of many different "types".
Dilettante, in post 2, gave you a link to all the different sub-types that can be in a Variant. However, be forewarned that VB6 can't necessarily deal with all of these sub-types. I'm sure I've missed something. Also, these aren't exactly the same words used to denote these in Dilettante's link, as these are the more VB6-friendly names of the sub-types. Also, a couple of the bits in these first two bytes are used to denote Array-or-not, and also ByRef-or-ByVal.
Now, so, the first two bytes are used to denote one of these sub-types. This leaves 14 more bytes. In most cases, the next 6 aren't used. So, we jump to the last 8 bytes. That link in post 3 that I gave you really gives you a great deal of detail on how these 8 bytes are mapped. If it's a ByVal Variant, the actual data is in these 8 bytes. If it's a ByRef Variant, an address to the data is in these 8 bytes.
For instance, Strings and Object are always ByRef. It should be obvious that we can't get most strings nor any object actually into these 8 bytes, so we "point" at them i. The same with arrays. However, actually, any of the sub-types could, in theory be a ByRef Variant. And that's about it. The only sub-type that actually uses all the remaining 14 bytes of a Variant is the Decimal type. This is a 14 byte structure that completely fills a Variant.
Without getting into details beyond the scope of your question, I'm not sure what more there is to say. Also, this has all really been said in the above, and references in links in the above. Variants aren't exclusive to VB6. So, your question should really be, "What's the structure of a Variant" with language not mattering. The structure of a Variant is the same regardless of language well, possibly with the exception of how Boolean "True" is handled, which is also discussed above.
Ahhh, I missed Boolean on my rattled off list. Last edited by Elroy; Nov 29th, at AM. No, there is no support in a Variant for any other form of boolean value than the VB6 Boolean type.
Originally Posted by dilettante No, there is no support in a Variant for any other form of boolean value than the VB6 Boolean type. Ahh, ok. Well, I guess the caveat should be then: When passing a Variant Boolean to another language, beware that it may not be the same as that language's Boolean, especially if it's a True value.
And I, in fact, already outlined the problem in post They are reserved fields. Arithmetic operations or functions can only be performed on numerical values. Variant variables with numbers require 16 bytes of memory.
Variables of the Variant data type with characters usually require 22 bytes of memory plus the memory required by the string.
For negative numbers: For positive numbers: 4. Reference Types User Defined Types Code Snippets Updated: 01 November 01 November
0コメント