Server side validations and unit testing in MVP
I think you have a typo in the last outside if statement, I assume you meant two different variables when you wrote this: 

if((!string.IsNullOrWhiteSpace(this.View.FromCustomerId) &amp;&amp; !string.IsNullOrWhiteSpace(this.View.FromCustomerId) &amp;&amp; (isValidFromCustomerId &amp;&amp; isValidFromCustomerId))
{ 
    if(parsedFromCustomerId &gt; parsedFromCustomerId)
    {
        errorCodes.Add("ERR_InvalidCustomerIdRange");
    }
}


parsedFromCustomerId will never be greater than parsedFromCustomerId

and the obvious one that won't cause a bug but probably won't give you the intended results is

 (isValidFromCustomerId &amp;&amp; isValidFromCustomerId)


I assume you mean to put a different boolean variable in there instead of the same one twice.

this code won't function, correctly.  

probably a Typo but you should proofread your code again just to make sure.