First of all you need to create an "ASP.NET Empty Web Site". Then use the following procedure.

Step 1

Create a ConvertDataTableToJson class in the App_Code folder and provide the following:

Convert DataTable To JSON String.       
    using System.Data 
    using System.Text; 
    public class ConvertDatatableToJson 
    { 
       public string DataTableToJson(DataTable dt) 
       { 
          DataSet ds = new DataSet(); 
          ds.Merge(dt); 
          StringBuilder JsonStr = new StringBuilder(); 
          if (ds != null && ds.Tables[0].Rows.Count > 0) 
          { 
             JsonStr.Append("["); 
             for (int i = 0; i < ds.Tables[0].Rows.Count; i++) 
             { 
                JsonStr.Append("{"); 
                for (int j = 0; j < ds.Tables[0].Columns.Count; j++) 
                { 
                   if (j < ds.Tables[0].Columns.Count - 1) 
                   { 
                      JsonStr.Append("\"" + ds.Tables[0].Columns[j].ColumnName.ToString() + "\":" + "\"" + ds.Tables[0].Rows[i][j].ToString() + "\","); 
                   } 
                   else if (j == ds.Tables[0].Columns.Count - 1) 
                   { 
                      JsonStr.Append("\"" + ds.Tables[0].Columns[j].ColumnName.ToString() + "\":" + "\"" + ds.Tables[0].Rows[i][j].ToString() + "\""); 
                   } 
                } 
                if (i == ds.Tables[0].Rows.Count - 1) 
                { 
                   JsonStr.Append("}"); 
                } 
                else    
                { 
                   JsonStr.Append("},"); 
                } 
             } 
             JsonStr.Append("]"); 
             return JsonStr.ToString(); 
          } 
          else 
          { 
             return null; 
          } 
       }

Step 2

Insert the grid view control into the Default.aspx page then write the following design code:

<asp:GridView ID="ui_grdVw_EmployeeDetail" runat="server" Width="50%" AutoGenerateColumns="false" HeaderStyle-CssClass="pageheading"> 
    <Columns> 
    <asp:TemplateField HeaderText="S.NO"> 
    <ItemTemplate> 
    <%#Container.DataItemIndex+1 %> 
    </ItemTemplate> 
    </asp:TemplateField> 
    <asp:TemplateField HeaderText="Employee ID"> 
    <ItemTemplate> 
    <asp:Label ID="ui_lbl_EmployeeID" runat="server" Text='<%# DataBinder.Eval(Container.DataItem, "Emp_id") %>'></asp:Label> 
    </ItemTemplate> 
    </asp:TemplateField> 
    <asp:TemplateField HeaderText="Employee Name"> 
    <ItemTemplate> 
    <asp:Label ID="ui_lbl_EmployeeName" runat="server" Text='<%# DataBinder.Eval(Container.DataItem, "Emp_Name") %>'></asp:Label> 
    </ItemTemplate> 
    </asp:TemplateField> 
    <asp:TemplateField HeaderText="Employee Post"> 
    <ItemTemplate> 
    <asp:Label ID="ui_lbl_EmpJob" runat="server" Text='<%# DataBinder.Eval(Container.DataItem, "Emp_job") %>'></asp:Label> 
    </ItemTemplate> 
    </asp:TemplateField> 
    <asp:TemplateField HeaderText="Department"> 
    <ItemTemplate> 
    <asp:Label ID="ui_lbl_Department" runat="server" Text='<%# DataBinder.Eval(Container.DataItem, "Emp_Dep") %>'></asp:Label> 
    </ItemTemplate> 
    </asp:TemplateField> 
    </Columns> 
    </asp:GridView> 
    <br /> 
    <asp:Button ID="ui_btn_Convert1" runat="server" Text="Manually Convert To Json" OnClick="ui_btn_Convert1_Click" /><br /><br /><br /> 
    <asp:Label ID="ui_lbl_JsonString1" runat="server"></asp:Label>

Step 3

Now, open the Deafult.asps.cs then write the following code:
    using System; 
    using System.Data; 
    public partial class _Default : System.Web.UI.Page 
    { 
       #region Global Variable 
       DataTable dt; 
       ConvertDatatableToJson dtJ; 
       string JsonString = string.Empty;    
       #endregion 
        
       protected void Page_Load(object sender, EventArgs e) 
       { 
          if (!IsPostBack) 
          { 
             ui_grdvw_EmployeeDetail_Bind(); 
          } 
       }       
       protected void ui_grdvw_EmployeeDetail_Bind() 
       { 
          dt = new DataTable(); 
          EmployeeRecord employeeRecord = new EmployeeRecord(); 
          dt = employeeRecord.EmpRecord(); 
          ViewState["dt"] = dt; 
          ui_grdVw_EmployeeDetail.DataSource = dt; 
          ui_grdVw_EmployeeDetail.DataBind(); 
       }       
       protected void ui_btn_Convert1_Click(object sender, EventArgs e) 
       { 
          dtJ = new ConvertDatatableToJson(); 
          JsonString = dtJ.DataTableToJson((DataTable)ViewState["dt"]); 
          ui_lbl_JsonString1.Text = JsonString; 
       } 
    }

Step 4

Press F5, run the project.

Now, convert the DataTable to a JSON string using the newtonsoft DLL.

Step 1

Download the Newtonsoft DLL and move it to the ASP.Net project's bin folder.

Step 2

Then, insert a button and label UI Control in the Deafult.aspx page as in the following:

<asp:Button ID="iu_btn_Convert2" runat="server" Text="Newtonsoft Convert To Json" OnClick="iu_btn_Convert2_Click" /> 
    <br /> 
    <br /> 
    <asp:Label ID="ui_lbl_JsonString2" runat="server"></asp:Label>

Step 3

Now, write the following code in Default.aspx.cs:
using this namespace

using Newtonsoft.Json;

And then:

protected void iu_btn_Convert2_Click(object sender, EventArgs e) 
    { 
       dt = (DataTable)ViewState["dt"]; 
       JsonString = JsonConvert.SerializeObject((DataTable)ViewState["dt"]); 
       ui_lbl_JsonString2.Text = JsonString; 
    }

Now, run the project and click on the Newtonsoft Convert to JSON     

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