gistfile1.cs

public static class AzureTableStringHelpers 
{
	public static string AsEncodedKey(this string key) 
	{ 
	    if (key == null) 
	    { 
	        return null; 
	    }

	    return Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(key)); 
	}

	public static string DecodeFromKey(this string encodedKey) 
	{ 
	    if (encodedKey == null) 
	    { 
	        return null; 
	    }

	    return System.Text.Encoding.UTF8.GetString(Convert.FromBase64String(encodedKey)); 
	}
}

/* The Tests */
[TestMethod] 
public void EncodeAndDecodeKey() 
{ 
    var encodedPartitionKey = "ADATUM\\Mary".AsEncodedKey();

    var actualPartitionKey = encodedPartitionKey.DecodeFromKey(); 
    Assert.AreEqual("ADATUM\\Mary", actualPartitionKey); 
}

/* Real World Usage */
var context = new ExpenseDataContext(this.account);
var query =
	(from expense in context.Expenses 
	 where expense.PartitionKey.CompareTo("ADATUM\\Mary".AsEncodedKey()) == 0 
	 select expense).AsTableServiceQuery();

return query.Execute();