C# Check the Empty or Null Value in All Datatable Values

Validate that DataTable cell is not null or empty

A null value would be returned as DBNull, and not a null literal.

var tab = new System.Data.DataTable();
tab.Columns.Add("c1");
tab.Rows.Add();

tab.Rows[0]["c1"] = null;
tab.Rows[0]["c1"].GetType() //returns DBNull

And as the DBNull.ToString() is guaranteed to return an empty string, you can safely use !String.IsNullOrEmpty(value.ToString())).

You can always guard against not being sure if an object is null by simply using null coalescing.

!String.IsNullOrEmpty(value?.ToString()))

Checking for null value with DataTable.Select

CopyToDataTable should fix the issue:

LV_Employees.DataSource = employees.Select("Name LIKE '%" + TB_Search.Text + "%'")
    .CopyToDataTable();
LV_Employees.DataBind();

How to get a value that is not null in a datatable column

Try this

  DataTable dt = new DataTable();

  dt.Columns.Add("emp_name", typeof(string));
  dt.Columns.Add("gender", typeof(string));
  dt.Columns.Add("Position", typeof(string));

  DataRow[] result = dt.Select("Position != null and Position != ''");
  foreach (DataRow row in result)
  {
      string position = row.Field<string>(2);
  }

How to check empty DataTable

If dataTable1 is null, it is not an empty datatable.

Simply wrap your foreach in an if-statement that checks if dataTable1 is null.
Make sure that your foreach counts over DataTable1.Rows or you will get a compilation error.

    if (dataTable1 != null)
    {
       foreach (DataRow dr in dataTable1.Rows)
       {
          // ...
       }
    }

How to determine a session variable contains data table is null or empty in C#

Your test is returning false because you are using a 'safe cast': as casting will return null if the conversion can't be made (see more on the C# guide here). Since you are trying to cast a datatable to a string, this will always return null, even when your session variable has a value.

To see the true nature of your problem, try converting them like this:

if(!string.IsNullOrEmpty((string)Session["tblItems"]))

This should thrown an exception, because you can't convert a datatable to a string like this. You will need to test for null like this:

if (Session["tblItems"] != null))

Update: having seen your comments, above, I've added a fuller example to explain what need to change. Your code should look more like this:

DataTable dt;

if (Session["tblItems"] != null))
{
    dt = (DataTable)Session["tblItems"];
    ...
}
else
{
    dt = new DataTable("tblItems1");
    ...
}

DataRow dr;
dr = dt.NewRow();
...

Note the changes I've made: initialise dt from session if there is a value, the cast will work in this case, and when it is null, construct a new one. You can then use the value in the section below without issue, because it will always have a value.

In addition, I would also recommend that you don't store your datatable in session at all, but rather just the values that you need.

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