Searches conditioned to sql from c #

You are assigned an instance of type System.Data.SqlClient.SqlCommand to a variable that is of type System.String

string sql = new SqlCommand("...");

Try this:

SqlCommand sqlCommand = new SqlCommand("sql aqui");

Note: So you can protect yourself from the sql injections I recommend the Dapper library. Very fast and easy to use.

You have the error in view:

  

can not be explicitly converted to string data from   System.Data.SqlClient.SqlCommand

What does this mean? what are you trying to turn something into, something it is not.

In your case, your code is:

string sql = new SqlCommand("SELECT * FROM Inventario WHERE Fecha BETWEEN '" + dtpdesde.Text 
+ "' AND '" + 
 dtphasta.Text + "'  AND [Descripcion de Articulo]= '" 
+ txtbusqueda.Text + "' AND Marca='" + txtmodelo.Text + "' 
AND [Tipo de Articulo]='" + cbtipo.SelectedItem + "'");

and the error itself tells you. SqlCommand is not a string, it is a SqlCommand object.

Unfortunately, you put an image, and I do not know what you tried to do next, but the main error is that sql has to be of type SqlCommand.