Article
11/18/2011

List<T>.TrimExcess Method

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Sets the capacity to the actual number of elements in the List<T>, if that number is less than a threshold value.

Namespace: System.Collections.Generic
Assembly: mscorlib (in mscorlib.dll)

Syntax

'Declaration
Public Sub TrimExcess

public void TrimExcess()

Remarks

This method can be used to minimize a collection's memory overhead if no new elements will be added to the collection. The cost of reallocating and copying a large List<T> can be considerable, however, so the TrimExcess method does nothing if the list is at more than 90 percent of capacity. This avoids incurring a large reallocation cost for a relatively small gain.

Note:
The current threshold of 90 percent might change in future releases.

This method is an O(n) operation, where n is Count.

To reset a List<T> to its initial state, call the Clear method before calling the TrimExcess method. Trimming an empty List<T> sets the capacity of the List<T> to the default capacity.

The capacity can also be set using the Capacity property.

Examples

The following code example demonstrates the TrimExcess method. Several properties and methods of the List<T> class are used to add, insert, and remove items from a list of strings. Then the TrimExcess method is used to reduce the capacity to match the count, and the Capacity and Count properties are displayed. If the unused capacity had been less than 10 percent of total capacity, the list would not have been resized. Finally, the contents of the list are cleared.

Imports System.Collections.Generic

Public Class Example

   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      Dim dinosaurs As New List(Of String)

      outputBlock.Text &= String.Format(vbLf & "Capacity: {0}", dinosaurs.Capacity) & vbCrLf

      dinosaurs.Add("Tyrannosaurus")
      dinosaurs.Add("Amargasaurus")
      dinosaurs.Add("Mamenchisaurus")
      dinosaurs.Add("Deinonychus")
      dinosaurs.Add("Compsognathus")

      outputBlock.Text &= vbCrLf
      For Each dinosaur As String In dinosaurs
         outputBlock.Text &= dinosaur & vbCrLf
      Next

      outputBlock.Text &= String.Format(vbLf & "Capacity: {0}", dinosaurs.Capacity) & vbCrLf
      outputBlock.Text &= String.Format("Count: {0}", dinosaurs.Count) & vbCrLf

      outputBlock.Text &= vbLf & String.Format("Contains(""Deinonychus"": {0}", _
          dinosaurs.Contains("Deinonychus")) & vbCrLf

      outputBlock.Text &= String.Format(vbLf & "Insert(2, ""Compsognathus"")") & vbCrLf
      dinosaurs.Insert(2, "Compsognathus")

      outputBlock.Text &= vbCrLf
      For Each dinosaur As String In dinosaurs
         outputBlock.Text &= dinosaur & vbCrLf
      Next

      outputBlock.Text &= String.Format(vbLf & "dinosaurs(3): {0}", dinosaurs(3)) & vbCrLf

      outputBlock.Text &= vbLf & "Remove(""Compsognathus"")" & vbCrLf
      dinosaurs.Remove("Compsognathus")

      outputBlock.Text &= vbCrLf
      For Each dinosaur As String In dinosaurs
         outputBlock.Text &= dinosaur & vbCrLf
      Next

      dinosaurs.TrimExcess()
      outputBlock.Text &= vbLf & "TrimExcess()" & vbCrLf
      outputBlock.Text &= String.Format("Capacity: {0}", dinosaurs.Capacity) & vbCrLf
      outputBlock.Text &= String.Format("Count: {0}", dinosaurs.Count) & vbCrLf

      dinosaurs.Clear()
      outputBlock.Text &= vbLf & "Clear()" & vbCrLf
      outputBlock.Text &= String.Format("Capacity: {0}", dinosaurs.Capacity) & vbCrLf
      outputBlock.Text &= String.Format("Count: {0}", dinosaurs.Count) & vbCrLf
   End Sub
End Class

' This code example produces the following output:
'
'Capacity: 0
'
'Tyrannosaurus
'Amargasaurus
'Mamenchisaurus
'Deinonychus
'Compsognathus
'
'Capacity: 8
'Count: 5
'
'Contains("Deinonychus"): True
'
'Insert(2, "Compsognathus")
'
'Tyrannosaurus
'Amargasaurus
'Compsognathus
'Mamenchisaurus
'Deinonychus
'Compsognathus
'
'dinosaurs(3): Mamenchisaurus
'
'Remove("Compsognathus")
'
'Tyrannosaurus
'Amargasaurus
'Mamenchisaurus
'Deinonychus
'Compsognathus
'
'TrimExcess()
'Capacity: 5
'Count: 5
'
'Clear()
'Capacity: 5
'Count: 0

using System;
using System.Collections.Generic;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      List<string> dinosaurs = new List<string>();

      outputBlock.Text += String.Format("\nCapacity: {0}", dinosaurs.Capacity) + "\n";

      dinosaurs.Add("Tyrannosaurus");
      dinosaurs.Add("Amargasaurus");
      dinosaurs.Add("Mamenchisaurus");
      dinosaurs.Add("Deinonychus");
      dinosaurs.Add("Compsognathus");

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      outputBlock.Text += String.Format("\nCapacity: {0}", dinosaurs.Capacity) + "\n";
      outputBlock.Text += String.Format("Count: {0}", dinosaurs.Count) + "\n";

      outputBlock.Text += String.Format("\nContains(\"Deinonychus\"): {0}",
          dinosaurs.Contains("Deinonychus")) + "\n";

      outputBlock.Text += String.Format("\nInsert(2, \"Compsognathus\")") + "\n";
      dinosaurs.Insert(2, "Compsognathus");

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      outputBlock.Text += String.Format("\ndinosaurs[3]: {0}", dinosaurs[3]) + "\n";

      outputBlock.Text += "\nRemove(\"Compsognathus\")" + "\n";
      dinosaurs.Remove("Compsognathus");

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      dinosaurs.TrimExcess();
      outputBlock.Text += "\nTrimExcess()" + "\n";
      outputBlock.Text += String.Format("Capacity: {0}", dinosaurs.Capacity) + "\n";
      outputBlock.Text += String.Format("Count: {0}", dinosaurs.Count) + "\n";

      dinosaurs.Clear();
      outputBlock.Text += "\nClear()" + "\n";
      outputBlock.Text += String.Format("Capacity: {0}", dinosaurs.Capacity) + "\n";
      outputBlock.Text += String.Format("Count: {0}", dinosaurs.Count) + "\n";
   }
}

/* This code example produces the following output:

Capacity: 0

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus

Capacity: 8
Count: 5

Contains("Deinonychus"): True

Insert(2, "Compsognathus")

Tyrannosaurus
Amargasaurus
Compsognathus
Mamenchisaurus
Deinonychus
Compsognathus

dinosaurs[3]: Mamenchisaurus

Remove("Compsognathus")

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus

TrimExcess()
Capacity: 5
Count: 5

Clear()
Capacity: 5
Count: 0
 */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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