Creating method to use one foreach instead multiple
First step is to look at the repeated code then copy an single into a method.
In this case a could method name would seem to be CreateDataTable.

What are the commonalities?


A ComboBox that has checked items.
Get an IQueryable or IEnumerable depending on which is returned by the where clause
Creates a list of the text value from each checked item
Creates a DataTable using ToStringDataTable(parameters


Since the type in and type out is all the same, this makes it much easier to refactor out into a method. The question then is what input and what output?

Input = ComboBox, output = `DataTable'. This give the method signature needed:

DataTable CreateDataTable(ComboBox cbo) {
}


Now put in those lines that are repeated and change the *Status to cbo and instead of assigning the DataTable just return the result of ToStringDataTable(parameters).

DataTable CreateDataTable(ComboBox cbo) {

    var checkedItems = cbo.CheckBoxItems.Where(x =&gt; x.Checked);
    List&lt;string&gt; parameterList = new List&lt;string&gt;();
    foreach (var i in checkedItms) {
        parameter.Add(i.Text);
    }

    return ToStringDataTable(parameters);
}


Now clean up the original method:

public class Refactor {
  private void BtnLoadReport_Click(object sender, EventArgs e)
    {
        var db = new SQLDataMgr();

        var designStatusParameters = CreateDataTable(cboDesignStatus);
        var shopStatusParameters = CreateDataTable(cboShopStatus);
        var customerTypeParameters = CreateDataTable(cboCustomerType);
        var customerParameters = CreateDataTable(cboCustomer);
        var resellerParameters = CreateDataTable(cboReseller);
        var stateParameters = CreateDataTable(cboState);
        var projectManagerParameters = CreateDataTable(cboProjectManager);
        var salesRepresentativeParameters = CreateDataTable(cboSalesRepresentative);
        //Rest of the method
    }   
}


This could be further improved by using some additional LINQ chains if the items returned by the where clause supports them.

Example:

List&lt;String&gt; parameters = cbo.CheckBoxItems.Where(x =&gt; x.Checked).Select(x =&gt; x.Text).ToList();


which would simplifying the method even further:

DataTable CreateDataTable(ComboBox cbo) {

    List&lt;string&gt; parameterList = List&lt;String&gt; parameters = cbo.CheckBoxItems.Where(x =&gt; x.Checked).Select(x =&gt; x.Text).ToList();

    return ToStringDataTable(parameters);
}