SQL Select with multiple parameters - asp.net

Question

I am trying to create a search page that would use multiple textboxes to build a query and output the results to the gridview but if the user only selects to complete some of the textboxes the select statement breaks and does not return results to the gridview. How do I cater for this? I tried only one variable but get a "Must declare the scalar variable " error.
ASP Code:
<h2>
Find Client</h2>
<div>
<fieldset class="login">
<legend>Client Detail</legend>
<div class="columns">
<div class="label">
<asp:Label ID="lblEmployeeNumber" CssClass="Smart-css_Label" runat="server">Client Number:</asp:Label>
</div>
<div class="textbox">
<asp:TextBox ID="txtClientNumber" placeholder="Client Number" runat="server" CssClass="Smart-css_Text"></asp:TextBox>
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
<asp:Label ID="lblName" CssClass="Smart-css_Label" runat="server">Name:</asp:Label>
</div>
<div class="textbox">
<asp:TextBox ID="txtName" placeholder="Name" runat="server" CssClass="Smart-css_Text"></asp:TextBox>
</div>
<div class="label">
<asp:Label ID="lblSurname" CssClass="Smart-css_Label" runat="server">Surname:</asp:Label>
</div>
<div class="textbox">
<asp:TextBox ID="txtSurname" placeholder="Surname" runat="server" CssClass="Smart-css_Text"></asp:TextBox>
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
<asp:Label ID="lblEmployer" CssClass="Smart-css_Label" runat="server">Client Type:</asp:Label>
</div>
<div class="textbox">
<asp:DropDownList ID="cboClientType" placeholder="Client Type" CssClass="Smart-css_Combo" runat="server">
</asp:DropDownList>
</div>
<div class="label">
<asp:Label ID="Label5" CssClass="Smart-css_Label" runat="server">Client Status:</asp:Label>
</div>
<div class="textbox">
<asp:DropDownList ID="cboClientStatus" CssClass="Smart-css_Combo" runat="server">
</asp:DropDownList>
</div>
<div class="label">
<asp:Label ID="Label4" CssClass="Smart-css_Label" runat="server">Company Name:</asp:Label>
</div>
<div class="textbox">
<asp:TextBox ID="txtCompanyName" runat="server" CssClass="Smart-css_Text"></asp:TextBox>
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
<asp:Label ID="Label13" CssClass="Smart-css_Label" runat="server">Postal Code:</asp:Label>
</div>
<div class="textbox">
<asp:TextBox ID="txtPostcode" runat="server" CssClass="Smart-css_Text"></asp:TextBox>
</div>
<div class="label">
<asp:Label ID="lblMobileNo" CssClass="Smart-css_Label" runat="server">Mobile Number:</asp:Label>
</div>
<div class="textbox">
<asp:TextBox ID="txtMobileNo" runat="server" CssClass="Smart-css_Text"></asp:TextBox>
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
</div>
<div class="textbox">
</div>
<div class="label">
<asp:Button ID="cmdSearch" CssClass="Smart-css_Button" runat="server" Text="Find Client"/>
</div>
<div class="textbox">
<asp:Button ID="cmdCancel" CssClass="Smart-css_Button" runat="server" Text="Cancel" />
</div>
</div>
</fieldset>
</div>
<div class= "widecolumns">
<asp:Label ID="lblMessage" runat="server" Text="Message" Visible="False"
Font-Bold="True" ForeColor="Red"></asp:Label>
</div>
<div class= "widecolumns">
<asp:GridView ID="grdclientslist" runat="server" AutoGenerateColumns="False"
CssClass="mGrid" PageSize="5">
</asp:GridView>
</div>
VB Code
Protected Sub cmdSearch_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cmdSearch.Click
Dim clientNumber As String = txtClientNumber.Text
Dim name As String = txtName.Text
Dim surname As String = txtSurname.Text
Dim clientType As String = cboClientType.Text
Dim clientStatus As String = cboClientStatus.Text
Dim companyName As String = txtCompanyName.Text
Dim postCode As String = txtPostcode.Text
Dim mobileNumber As String = txtMobileNo.Text
sqlconn.SQLCmd.Parameters.AddWithValue("#clientNumber", clientNumber)
sqlconn.SQLCmd.Parameters.AddWithValue("#name", name)
sqlconn.SQLCmd.Parameters.AddWithValue("#surname", surname)
sqlconn.SQLCmd.Parameters.AddWithValue("#clientType", clientType)
sqlconn.SQLCmd.Parameters.AddWithValue("#clientStatus", clientStatus)
sqlconn.SQLCmd.Parameters.AddWithValue("#companyName", companyName)
sqlconn.SQLCmd.Parameters.AddWithValue("#postCode", postCode)
sqlconn.SQLCmd.Parameters.AddWithValue("#mobileNumber", mobileNumber)
sqlconn.ExecQuery("SELECT * FROM epm_ClientDetail WHERE (client_ID LIKE '%' + #clientNumber + '%') OR (client_Name LIKE '%' + #name + '%') OR (client_Surname LIKE '%' + #surname + '%') OR (client_Type LIKE '%' + #clientType + '%') OR (client_Status LIKE '%' + #clientStatus + '%') OR (client_CompanyName LIKE '%' + #companyName + '%') OR (client_StreetPostCode LIKE '%' + #postCode + '%') OR (client_MobileNo LIKE '%' + #mobileNumber + '%')")
If String.IsNullOrEmpty(sqlconn.Exception) Then
grdclientslist.DataSource = sqlconn.SQLDS.Tables(0)
End If
End Sub