What is the IsFixedSize property of Hashtable class in C#?

The IsFixedSize property of the Hashtable class in C# returns a bool value indicating whether the Hashtable has a fixed size. When this property returns false, you can add or remove elements. When it returns true, the size is fixed and you cannot add or remove elements.

For a regular Hashtable created using the default constructor, IsFixedSize always returns false, meaning it can grow dynamically as you add elements.

Syntax

Following is the syntax to check if a Hashtable has a fixed size −

bool isFixed = hashtable.IsFixedSize;

Return Value

The IsFixedSize property returns −

• true − if the Hashtable has a fixed size
• false − if the Hashtable can grow or shrink dynamically

Using IsFixedSize with Regular Hashtable

Example

using System;
using System.Collections;

class Program {
   static void Main(string[] args) {
      Hashtable ht = new Hashtable();
      ht.Add("D01", "Finance");
      ht.Add("D02", "HR");
      ht.Add("D03", "Operations");
      
      Console.WriteLine("IsFixedSize = " + ht.IsFixedSize);
      Console.WriteLine("Count = " + ht.Count);
      
      // Add more elements to verify it can grow
      ht.Add("D04", "Marketing");
      Console.WriteLine("After adding D04:");
      Console.WriteLine("Count = " + ht.Count);
      Console.WriteLine("IsFixedSize = " + ht.IsFixedSize);
   }
}

The output of the above code is −

IsFixedSize = False
Count = 3
After adding D04:
Count = 4
IsFixedSize = False

Creating a Fixed-Size Hashtable

You can create a fixed-size Hashtable using the Hashtable.Synchronized() method with a read-only wrapper or by using collection wrappers −

Example

using System;
using System.Collections;

class Program {
   static void Main(string[] args) {
      Hashtable originalHt = new Hashtable();
      originalHt.Add("A", "Apple");
      originalHt.Add("B", "Banana");
      
      // Create a read-only wrapper
      Hashtable readOnlyHt = Hashtable.ReadOnly(originalHt);
      
      Console.WriteLine("Original Hashtable:");
      Console.WriteLine("IsFixedSize = " + originalHt.IsFixedSize);
      Console.WriteLine("IsReadOnly = " + originalHt.IsReadOnly);
      
      Console.WriteLine("\nRead-only Hashtable:");
      Console.WriteLine("IsFixedSize = " + readOnlyHt.IsFixedSize);
      Console.WriteLine("IsReadOnly = " + readOnlyHt.IsReadOnly);
   }
}

The output of the above code is −

Original Hashtable:
IsFixedSize = False
IsReadOnly = False

Read-only Hashtable:
IsFixedSize = True
IsReadOnly = True

Comparison

Conclusion

The IsFixedSize property helps determine whether you can modify a Hashtable by adding or removing elements. Regular Hashtable instances return false, while read-only wrappers return true. This property is useful for validating collection capabilities before attempting modifications.