Extension method must be defined in a non-generic static class

Question

I have written the following code to find out the missing sequence but i am getting the error as i mentioned this is my code

public partial class Missing : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
    List<int> daysOfMonth =
       new List<int>() { 6, 2, 4, 1, 9, 7, 3, 10, 15, 19, 11, 18, 13, 22, 24, 20, 27, 31, 25, 28 };
    Response.Write("List of days:");
    foreach (var num in daysOfMonth)
    {
        Response.Write(num);
    }
    Response.Write("\n\nMissing days are: ");
    // Calling the Extension Method in the List of type int 
    foreach (var number in daysOfMonth.FindMissing()){Response.Write(number);}
}
public static IEnumerable<int> FindMissing(this List<int> list)
{
    // Sorting the list
    list.Sort();
    // First number of the list
    var firstNumber = list.First();
    // Last number of the list
    var lastNumber = list.Last();
    // Range that contains all numbers in the interval
    // [ firstNumber, lastNumber ]
    var range = Enumerable.Range(firstNumber, lastNumber - firstNumber);
    // Getting the set difference
    var missingNumbers = range.Except(list);
    return missingNumbers;
}

}

I am getting the error as follows Extension method must be defined in a non-generic static class can any one help me

Possible duplicate of [Extension methods must be defined in a non-generic static class](https://stackoverflow.com/questions/6096299/extension-methods-must-be-defined-in-a-non-generic-static-class) — amin, Aug 21 '17 at 05:35

score 21 · Accepted Answer · answered Feb 11 '12 at 05:46

21

As the error states, extension methods can only be declared on a non-generic static class. You are attempting to declare the FindMissing method in the Missing class, which is not a non-generic static class.

You have two options:

Make the method a normal method, in which case it can stay in the Missing class
Declare another class, perhaps MissingExtensions, to contain the method

This is what the second option would look like:

public static class MissingExtensions
{
    public static IEnumerable<int> FindMissing(this List<int> list)
    {
        // ...
    }
}

answered Feb 11 '12 at 05:46

Bryan Watts

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I am getting this error if i write as per you said `Extension method must be defined in a top level static class; MissingExtensions is a nested class` – Chaitanya Feb 11 '12 at 06:12
@Chaitanya You didn't write it as he said; he said nothing about nesting it inside the outer class. – Andrew Barber Feb 11 '12 at 06:48

score 4 · Answer 2 · answered Feb 11 '12 at 08:35

This is what you have to write as per Bryan watts answer

public partial class Missing : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
     // Your code
}
}

public static class MissingExtensions
{
    public static IEnumerable<int> FindMissing(this List<int> list)
    {
        // ...
    }
}

Extension method must be defined in a non-generic static class

2 Answers2

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