How to make search between two dates accept null not obligatory search proplem

Question

Hi guys when i search record between two dates it works ok success but you must enter date from and dateto first to  to make search
i will show what i need from this example
I need to search dynamic by 4 textbox
1-datefrom
2-dateto
3-EmployeeNo
4-EmployeeName
but search i need must be dynamic meaning
if i enter employee no only give me employee no found in database
if i enter employee name give me employees found with this name using like
if i enter all 4 text box null and enter button search get all data
but i have proplem in this query when i need to search by click search button
i must write date from and date to firstly then write employee no or employee name if i need to search
so that i need to search by employee no alone or employee name alone without using date from and date to
And if i search without using datefrom and dateto it give me message error 'string wasnot recognized as valid datetime"
my stored procedure and code as following :
ALTER proc [dbo].[CollectsearchData]
@StartDate datetime,
@EndDate datetime,
@EmployeeID  NVARCHAR(50),
@EmployeeName  nvarchar(50)
as
Begin
Declare @SQLQuery as nvarchar(2000)
SET @SQLQuery ='SELECT * from ViewEmployeeTest Where (1=1)'
If (@StartDate is not NULL)
Set @SQLQuery = @SQLQuery + ' And (joindate >= '''+ Cast(@StartDate as varchar(100))+''')'
If (@EndDate is not NULL)
Set @SQLQuery = @SQLQuery + ' And (joindate <= '''+ Cast(@EndDate as varchar(100))+''')' 
If @EmployeeID <>''
Set @SQLQuery = @SQLQuery + 'And (EmployeeID = '+ @EmployeeID+') '
If @EmployeeName Is Not Null
Set @SQLQuery = @SQLQuery + ' AND (DriverName LIKE
''%'+@EmployeeName+'%'') '
Print @sqlQuery
Exec (@SQLQuery) 
End
Function using
public DataTable SearchDataA(string ConnectionString,string EmployeeNo,string EmployeeName, DateTime StartDate, DateTime EndDate)
SqlConnection con = new SqlConnection(ConnectionString);
SqlCommand cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.StoredProcedure;
cmd.CommandText = "CollectsearchData";//work
cmd.Parameters.Add("@StartDate", SqlDbType.DateTime);
cmd.Parameters.Add("@EndDate", SqlDbType.DateTime);
cmd.Parameters.Add("@EmployeeID", SqlDbType.NVarChar, 50);
cmd.Parameters.Add("@EmployeeName", SqlDbType.NVarChar, 50);
cmd.Parameters["@StartDate"].Value = StartDate;
cmd.Parameters["@EndDate"].Value = EndDate;
cmd.Parameters["@EmployeeID"].Value = EmployeeNo;
cmd.Parameters["@EmployeeName"].Value = EmployeeName;
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = cmd;
DataSet ds = new DataSet();
da.Fill(ds);
DataTable dt = ds.Tables[0];
return dt;
interface button search
 try
CultureInfo ukCulture = new CultureInfo("en-GB");             
FleetManagment.Fleet fleet = new FleetManagment.Fleet();
DataTable Table = fleet.SearchDataA("Data Source=" + value1 + ";Initial Catalog=" + value2 + ";User ID=" + value3 + ";Password=" + value4 + "",textBox3.Text,textBox4.Text, DateTime.Parse(textBox1.Text,
ukCulture.DateTimeFormat), Convert.ToDateTime(textBox2.Text, ukCulture.DateTimeFormat));
dataGridView1.DataSource = Table;
dataGridView1.Refresh();
catch (Exception ex)
MessageBox.Show(ex + "error");

Answer 1

Yes, the below code should not be passed any value: (I am not sure of the syntax in .NET,Sorry) 
--If startdate len is 0 - do not assign this value
cmd.Parameters["@StartDate"].Value = StartDate;
--If endate len is 0 - do not assign this value
cmd.Parameters["@EndDate"].Value = EndDate;