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Learn more about: For Each...Next Statement (Visual Basic) |
For Each...Next Statement |
07/20/2015 |
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ebce3120-95c3-42b1-b70b-fa7da40c75e2 |
Repeats a group of statements for each element in a collection.
For Each element [ As datatype ] In group
[ statements ]
[ Continue For ]
[ statements ]
[ Exit For ]
[ statements ]
Next [ element ]| Term | Definition |
|---|---|
element |
Required in the For Each statement. Optional in the Next statement. Variable. Used to iterate through the elements of the collection. |
datatype |
Optional if Option Infer is on (the default) or element is already declared; required if Option Infer is off and element isn't already declared. The data type of element. |
group |
Required. A variable with a type that's a collection type or Object. Refers to the collection over which the statements are to be repeated. |
statements |
Optional. One or more statements between For Each and Next that run on each item in group. |
Continue For |
Optional. Transfers control to the start of the For Each loop. |
Exit For |
Optional. Transfers control out of the For Each loop. |
Next |
Required. Terminates the definition of the For Each loop. |
Use a For Each...Next loop when you want to repeat a set of statements for each element of a collection or array.
Tip
A For...Next Statement works well when you can associate each iteration of a loop with a control variable and determine that variable's initial and final values. However, when you are dealing with a collection, the concept of initial and final values isn't meaningful, and you don't necessarily know how many elements the collection has. In this kind of case, a For Each...Next loop is often a better choice.
In the following example, the For Each…Next statement iterates through all the elements of a List collection.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Examples.vb" id="init":::
For more examples, see Collections and Arrays.
You can nest For Each loops by putting one loop within another.
The following example demonstrates nested For Each…Next structures.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Examples.vb" id="nested":::
When you nest loops, each loop must have a unique element variable.
You can also nest different kinds of control structures within each other. For more information, see Nested Control Structures.
The Exit For statement causes execution to exit the For…Next loop and transfers control to the statement that follows the Next statement.
The Continue For statement transfers control immediately to the next iteration of the loop. For more information, see Continue Statement.
The following example shows how to use the Continue For and Exit For statements.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Examples.vb" id="exitcontinue":::
You can put any number of Exit For statements in a For Each loop. When used within nested For Each loops, Exit For causes execution to exit the innermost loop and transfers control to the next higher level of nesting.
Exit For is often used after an evaluation of some condition, for example, in an If...Then...Else structure. You might want to use Exit For for the following conditions:
-
Continuing to iterate is unnecessary or impossible. This might be caused by an erroneous value or a termination request.
-
An exception is caught in a
Try...Catch...Finally. You might useExit Forat the end of theFinallyblock. -
There an endless loop, which is a loop that could run a large or even infinite number of times. If you detect such a condition, you can use
Exit Forto escape the loop. For more information, see Do...Loop Statement.
You use an iterator to perform a custom iteration over a collection. An iterator can be a function or a Get accessor. It uses a Yield statement to return each element of the collection one at a time.
You call an iterator by using a For Each...Next statement. Each iteration of the For Each loop calls the iterator. When a Yield statement is reached in the iterator, the expression in the Yield statement is returned, and the current location in code is retained. Execution is restarted from that location the next time that the iterator is called.
The following example uses an iterator function. The iterator function has a Yield statement that's inside a For…Next loop. In the ListEvenNumbers method, each iteration of the For Each statement body creates a call to the iterator function, which proceeds to the next Yield statement.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Examples.vb" id="iterator":::
For more information, see Iterators, Yield Statement, and Iterator.
When a For Each…Next statement runs, Visual Basic evaluates the collection only one time, before the loop starts. If your statement block changes element or group, these changes don't affect the iteration of the loop.
When all the elements in the collection have been successively assigned to element, the For Each loop stops and control passes to the statement following the Next statement.
If Option Infer is on (its default setting), the Visual Basic compiler can infer the data type of element. If it is off and element hasn't been declared outside the loop, you must declare it in the For Each statement. To declare the data type of element explicitly, use an As clause. Unless the data type of element is defined outside the For Each...Next construct, its scope is the body of the loop. Note that you cannot declare element both outside and inside the loop.
You can optionally specify element in the Next statement. This improves the readability of your program, especially if you have nested For Each loops. You must specify the same variable as the one that appears in the corresponding For Each statement.
You might want to avoid changing the value of element inside a loop. Doing this can make it more difficult to read and debug your code. Changing the value of group doesn't affect the collection or its elements, which were determined when the loop was first entered.
When you're nesting loops, if a Next statement of an outer nesting level is encountered before the Next of an inner level, the compiler signals an error. However, the compiler can detect this overlapping error only if you specify element in every Next statement.
If your code depends on traversing a collection in a particular order, a For Each...Next loop isn't the best choice, unless you know the characteristics of the enumerator object the collection exposes. The order of traversal isn't determined by Visual Basic, but by the xref:System.Collections.IEnumerator.MoveNext%2A method of the enumerator object. Therefore, you might not be able to predict which element of the collection is the first to be returned in element, or which is the next to be returned after a given element. You might achieve more reliable results using a different loop structure, such as For...Next or Do...Loop.
The runtime must be able to convert the elements in group to element. The [Option Strict] statement controls whether both widening and narrowing conversions are allowed (Option Strict is off, its default value), or whether only widening conversions are allowed (Option Strict is on). For more information, see Narrowing conversions.
The data type of group must be a reference type that refers to a collection or an array that's enumerable. Most commonly this means that group refers to an object that implements the xref:System.Collections.IEnumerable interface of the System.Collections namespace or the xref:System.Collections.Generic.IEnumerable%601 interface of the System.Collections.Generic namespace. System.Collections.IEnumerable defines the xref:System.Collections.IEnumerable.GetEnumerator%2A method, which returns an enumerator object for the collection. The enumerator object implements the System.Collections.IEnumerator interface of the System.Collections namespace and exposes the xref:System.Collections.IEnumerator.Current%2A property and the xref:System.Collections.IEnumerator.Reset%2A and xref:System.Collections.IEnumerator.MoveNext%2A methods. Visual Basic uses these to traverse the collection.
When Option Strict is set to On, narrowing conversions ordinarily cause compiler errors. In a For Each statement, however, conversions from the elements in group to element are evaluated and performed at run time, and compiler errors caused by narrowing conversions are suppressed.
In the following example, the assignment of m as the initial value for n doesn't compile when Option Strict is on because the conversion of a Long to an Integer is a narrowing conversion. In the For Each statement, however, no compiler error is reported, even though the assignment to number requires the same conversion from Long to Integer. In the For Each statement that contains a large number, a run-time error occurs when xref:Microsoft.VisualBasic.CompilerServices.Conversions.ToInteger%2A is applied to the large number.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Program.vb":::
When execution of a For Each...Next loop starts, Visual Basic verifies that group refers to a valid collection object. If not, it throws an exception. Otherwise, it calls the xref:System.Collections.IEnumerator.MoveNext%2A method and the xref:System.Collections.IEnumerator.Current%2A property of the enumerator object to return the first element. If MoveNext indicates that there is no next element, that is, if the collection is empty, the For Each loop stops and control passes to the statement following the Next statement. Otherwise, Visual Basic sets element to the first element and runs the statement block.
Each time Visual Basic encounters the Next statement, it returns to the For Each statement. Again it calls MoveNext and Current to return the next element, and again it either runs the block or stops the loop depending on the result. This process continues until MoveNext indicates that there is no next element or an Exit For statement is encountered.
Modifying the Collection. The enumerator object returned by xref:System.Collections.IEnumerable.GetEnumerator%2A normally doesn't let you change the collection by adding, deleting, replacing, or reordering any elements. If you change the collection after you have initiated a For Each...Next loop, the enumerator object becomes invalid, and the next attempt to access an element causes an xref:System.InvalidOperationException exception.
However, this blocking of modification isn't determined by Visual Basic, but rather by the implementation of the xref:System.Collections.IEnumerable interface. It is possible to implement IEnumerable in a way that allows for modification during iteration. If you are considering doing such dynamic modification, make sure that you understand the characteristics of the IEnumerable implementation on the collection you are using.
Modifying Collection Elements. The xref:System.Collections.IEnumerator.Current%2A property of the enumerator object is ReadOnly, and it returns a local copy of each collection element. This means that you cannot modify the elements themselves in a For Each...Next loop. Any modification you make affects only the local copy from Current and isn't reflected back into the underlying collection. However, if an element is a reference type, you can modify the members of the instance to which it points. The following example modifies the BackColor member of each thisControl element. You cannot, however, modify thisControl itself.
Sub LightBlueBackground(thisForm As System.Windows.Forms.Form)
For Each thisControl In thisForm.Controls
thisControl.BackColor = System.Drawing.Color.LightBlue
Next thisControl
End SubThe previous example can modify the BackColor member of each thisControl element, although it cannot modify thisControl itself.
Traversing Arrays. Because the xref:System.Array class implements the xref:System.Collections.IEnumerable interface, all arrays expose the xref:System.Array.GetEnumerator%2A method. This means that you can iterate through an array with a For Each...Next loop. However, you can only read the array elements. You cannot change them.
The following example lists all the folders in the C:\ directory by using the xref:System.IO.DirectoryInfo class.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Examples.vb" id="foreachdir":::
The following example illustrates a procedure for sorting a collection. The example sorts instances of a Car class that are stored in a xref:System.Collections.Generic.List%601. The Car class implements the xref:System.IComparable%601 interface, which requires that the xref:System.IComparable%601.CompareTo%2A method be implemented.
Each call to the xref:System.IComparable%601.CompareTo%2A method makes a single comparison that's used for sorting. User-written code in the CompareTo method returns a value for each comparison of the current object with another object. The value returned is less than zero if the current object is less than the other object, greater than zero if the current object is greater than the other object, and zero if they are equal. This enables you to define in code the criteria for greater than, less than, and equal.
In the ListCars method, the cars.Sort() statement sorts the list. This call to the xref:System.Collections.Generic.List%601.Sort%2A method of the xref:System.Collections.Generic.List%601 causes the CompareTo method to be called automatically for the Car objects in the List.
:::code language="vb" source="./snippets/for-each-next-statement/vb/Examples.vb" id="sort":::