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Returns the result of explicitly converting an expression to a specified data type, object, structure, class, or interface.
CType(expression, typename)
expression
Any valid expression. If the value of expression
is outside the range allowed by typename
, Visual Basic throws an exception.
typename
Any expression that is legal within an As
clause in a Dim
statement, that is, the name of any data type, object, structure, class, or interface.
Съвет
You can also use the following functions to perform a type conversion:
CByte
, CDbl
, and CInt
that perform a conversion to a specific data type. For more information, see Type Conversion Functions.CType
when converting to and from the Object
data type.CType
is compiled inline, which means that the conversion code is part of the code that evaluates the expression. In some cases, the code runs faster because no procedures are called to perform the conversion.
If no conversion is defined from expression
to typename
(for example, from Integer
to Date
), Visual Basic displays a compile-time error message.
If a conversion fails at run time, the appropriate exception is thrown. If a narrowing conversion fails, an OverflowException is the most common result. If the conversion is undefined, an InvalidCastException in thrown. For example, this can happen if expression
is of type Object
and its run-time type has no conversion to typename
.
If the data type of expression
or typename
is a class or structure you've defined, you can define CType
on that class or structure as a conversion operator. This makes CType
act as an overloaded operator. If you do this, you can control the behavior of conversions to and from your class or structure, including the exceptions that can be thrown.
A comparison of the type conversion keywords is as follows.
Keyword | Data types | Argument relationship | Run-time failure |
---|---|---|---|
CType |
Any data types | Widening or narrowing conversion must be defined between the two data types | Throws InvalidCastException |
DirectCast Operator | Any data types | One type must inherit from or implement the other type | Throws InvalidCastException |
TryCast | Reference types only | One type must inherit from or implement the other type | Returns Nothing |
The CType
operator can also be overloaded on a class or structure defined outside your code. If your code converts to or from such a class or structure, be sure you understand the behavior of its CType
operator. For more information, see Operator Procedures.
Type conversions of dynamic objects are performed by user-defined dynamic conversions that use the TryConvert or BindConvert methods. If you're working with dynamic objects, use the CTypeDynamic method to convert the dynamic object.
The following example uses the CType
function to convert an expression to the Single
data type.
Dim testNumber As Long = 1000
' The following line of code sets testNewType to 1000.0.
Dim testNewType As Single = CType(testNumber, Single)
For additional examples, see Implicit and Explicit Conversions.
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Създаване на интелигентни приложения
17.03, 23 ч. - 21.03, 23 ч.
Присъединете се към поредицата срещи, за да изградите мащабируеми AI решения, базирани на реални случаи на употреба с колеги разработчици и експерти.
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