calling a c# class method from html page

You can't call normal method. The method must be static and web method.

Try this:

public class Crud
{
    [WebMethod]
    public static String generateFiles(String name)
    {

        return(generateHtml(name));
    }
     private String generateHtml(String name)
    {

       var filename = "C:\temp\"" + name + ".html";

        try
        {
            FileStream fs = new FileStream(filename, FileMode.Create);
            return "True";
        }
        catch(Exception e)
        {
            return e.ToString();
        }

    }
}

You are missing a controler in your project.

You try to retrieve data from a cs file without a controler (or a [WebMethod])? this is impossible.

Try looking for some MVC guides, Here is one from microsoft web site

You dont have to use all that ASP component that showen there, but you can see there how to retrieve data from the server to the client.

Basic syntax

<script type="text/javascript">             //Default.aspx
   function myfunction() {     
             $.ajax({
             type: "POST",
             url: 'Default.aspx/myfunction',
             data: "mystring",
             contentType: "application/json; charset=utf-8",
             dataType: "json",
             success: function (msg) {
                 alert("success");
             },
             error: function (e) {
                 $("#divResult").html("Something Wrong.");
             }
         });
     }

Default.aspx.cs

[WebMethod]
public static String myfunction(string name)
{
    return "Your String"
}

If you want to use page call without ajax: Ref

//cs file (code behind)
[ScriptMethod, WebMethod]
public static string GetLabelText(string param1)
{
   return "Hello";
}
//aspx page
 <script type="text/javascript">
  function InsertLabelData() {
      PageMethods.GetLabelText(param1,onSuccess, onFailure);
  }

  function onSuccess(result) {
      var lbl = document.getElementById(‘lbl’);
      lbl.innerHTML = result;
  }

  function onFailure(error) {
      alert(error);
  }
  InsertLabelData();
</script>

If you're using ASP.NET Web Forms, there is a WebMethodAttribute you can use instead of calling .cs file directly which unsupported by AJAX due to no URL routing enabled for normal classes. The web method must be declared as static:

// if you're using ASMX web service, change to this class declaration:
// public class Crud : System.Web.Services.WebService
public class Crud : System.Web.UI.Page
{

    [System.Web.Services.WebMethod]
    public static String generateFiles(String name)
    {
        return generateHtml(name);
    }

    private String generateHtml(String name)
    {
        var filename = "C:\temp\"" + name + ".html";

        try
        {
            FileStream fs = new FileStream(filename, FileMode.Create);
            return "True";
        }
        catch(Exception e)
        {
            return e.ToString();
        }
    }
}

Then your AJAX call URL should be changed to this (note that the web method should be exist in code-behind file, e.g. Crud.aspx.cs or Crud.asmx.cs):

$.ajax({
          type: "POST",
          url: "Crud.aspx/generateFiles", // web service uses .asmx instead of .aspx
          data: { name: name }, // use JSON.stringify if you're not sure
          contentType: "application/json; charset=utf-8",
          dataType: "json",
          success: function (val) {
             alert(val);
             if (val == 0) {
                $("#feedbackMsg").html("Success");
             }
             else
             {
                 $("#feedbackMsg").html("Sorry cannot perform such operation");
             }
          },
          error: function (e) {
             $("#feedbackMsg").html("Something Wrong.");
          }
});

If ASP.NET MVC is used, use JsonResult to return JSON string as success result:

public class CrudController : Controller
{
    [HttpPost]
    public JsonResult generateFiles(String name)
    {
        return Json(generateHtml(name));
    }
}

The AJAX call for the action method looks similar but the URL part is slightly different:

$.ajax({
          type: "POST",
          url: "Crud/generateFiles",
          data: { name: name }, // use JSON.stringify if you're not sure
          contentType: "application/json; charset=utf-8",
          dataType: "json",
          success: function (val) {
             alert(val);
             if (val == 0) {
                $("#feedbackMsg").html("Success");
             }
             else
             {
                 $("#feedbackMsg").html("Sorry cannot perform such operation");
             }
          },
          error: function (e) {
             $("#feedbackMsg").html("Something Wrong.");
          }
});

    //Perfect solution
  var params = { param1: value, param2: value2}
    $.ajax({
        type: 'POST',
        contentType: 'application/json; charset=utf-8',
        url: '../aspxPage.aspx/methodName',
        data: JSON.stringify(params),
        datatype: 'json',
        success: function (data) {
       var  MethodReturnValue = data.d

        },
        error: function (xmlhttprequest, textstatus, errorthrown) {
            alert(" conection to the server failed ");
        }
    });

//PLEASE menntion [WebMethod] attribute on your method.