This is a sample code named ADOSample1 which is not working correctly. (in 3rd Edition)

Need to find a node using ProductName in the following xml file

<?xml version="1.0" standalone="yes"?>
<XMLProducts>
  <products>
    <ProductID>1</ProductID>
    <ProductName>Chai</ProductName>
    <SupplierID>1</SupplierID>
    <CategoryID>1</CategoryID>
    <QuantityPerUnit>10 boxes x 20 bags</QuantityPerUnit>
    <UnitPrice>18</UnitPrice>
    <UnitsInStock>39</UnitsInStock>
    <UnitsOnOrder>0</UnitsOnOrder>
    <ReorderLevel>10</ReorderLevel>
    <Discontinued>false</Discontinued>
  </products>
  <products>
    <ProductID>2</ProductID>
    <ProductName>Chang</ProductName>
    <SupplierID>1</SupplierID>
    <CategoryID>1</CategoryID>
    <QuantityPerUnit>24 - 12 oz bottles</QuantityPerUnit>
    <UnitPrice>19</UnitPrice>
    <UnitsInStock>17</UnitsInStock>
    <UnitsOnOrder>40</UnitsOnOrder>
    <ReorderLevel>25</ReorderLevel>
    <Discontinued>false</Discontinued>
  </products>
  <products>
    <ProductID>3</ProductID>
    <ProductName>Aniseed Syrup</ProductName>
    <SupplierID>1</SupplierID>
    <CategoryID>2</CategoryID>
    <QuantityPerUnit>12 - 550 ml bottles</QuantityPerUnit>
    <UnitPrice>10</UnitPrice>
    <UnitsInStock>13</UnitsInStock>
    <UnitsOnOrder>70</UnitsOnOrder>
    <ReorderLevel>25</ReorderLevel>
    <Discontinued>false</Discontinued>
  </products>
  <products>
    <ProductID>4</ProductID>
    <ProductName>Chef Anton's Cajun Seasoning</ProductName>
    <SupplierID>2</SupplierID>
    <CategoryID>2</CategoryID>
    <QuantityPerUnit>48 - 6 oz jars</QuantityPerUnit>
    <UnitPrice>22</UnitPrice>
    <UnitsInStock>53</UnitsInStock>
    <UnitsOnOrder>0</UnitsOnOrder>
    <ReorderLevel>0</ReorderLevel>
    <Discontinued>false</Discontinued>
  </products>
  <products>
    <ProductID>5</ProductID>
    <ProductName>Chef Anton's Gumbo Mix</ProductName>
    <SupplierID>2</SupplierID>
    <CategoryID>2</CategoryID>
    <QuantityPerUnit>36 boxes</QuantityPerUnit>
    <UnitPrice>21.35</UnitPrice>
    <UnitsInStock>0</UnitsInStock>
    <UnitsOnOrder>0</UnitsOnOrder>
    <ReorderLevel>0</ReorderLevel>
    <Discontinued>true</Discontinued>
  </products>
</XMLProducts>

The code is the following:
private void listBox1_SelectedIndexChanged(object sender, System.EventArgs e)
        {
            //when you click on the listbox,
            //a message box appears with the unit price
            string srch = "XMLProducts/products[ProductName='"+
                listBox1.SelectedItem.ToString()+"']";
            //string srch="XMLProducts/Products[ProductName=" + '"' + listBox1.SelectedItem.ToString() + '"' + "]";
            XmlNode foundNode = doc.SelectSingleNode(srch);
            if(foundNode!=null)
                MessageBox.Show(foundNode.SelectSingleNode("UnitPr ice").InnerText);
            else
                MessageBox.Show("Not found");
        }

Using "XMLProducts/products[ProductName='"+ listBox1.SelectedItem.ToString()+"']";
works well for <ProductName>Aniseed Syrup</ProductName>

However, for ProductName which has ' mark within (eg, <ProductName>Chef Anton's Cajun Seasoning</ProductName>), this expression doesn't work. The sample code originally writes "XMLProducts/Products[ProductName=" + '"' + listBox1.SelectedItem.ToString() + '"' + "]";
which does not work at all!!!

What's the right expression for this situation?

Addtional notes:
Using the sample Xpath "XMLProducts/Products[ProductName=" + '"' + listBox1.SelectedItem.ToString() + '"' + "]"
the result for <ProductName>Chef Anton's Cajun Seasoning</ProductName> is "XMLProducts/Products[ProductName=\"Chef Anton's Cajun Seasoning\"]"