Accessing properties by a string name

This morning Julian Voelcker came to me with an interesting issue that I’ve looked into before but I’ve never really looked into a re-useable solution. Seeing as it’s fun Friday I thought why not ;)

The scenario: I would like to offer my users a custom mail merge facility where by they can insert values stored in the database such as their name. The selection of columns is unlikely to be changed and if it does then I’ll be the one to do it. There are about 20 fields to choose from.

Easy enough, in the past I’ve kept it to a minimum and then just done a simple find and replace on the body i.e.:

//Create a dataset and add some test columns
DataTable dt = new DataTable();
dt.Columns.Add("Name");
dt.Columns.Add("Email");

Add some test data

#region Add some test data

DataRow dr = dt.NewRow();
dr["Name"] = "Julian";
dr["Email"] = "[email protected]";
dt.Rows.Add(dr);

dr = dt.NewRow();
dr["Name"] = "Tim";
dr["Email"] = "[email protected]";
dt.Rows.Add(dr);

#endregion

Create the example email body

#region Create the example email body

string emailBody = "<p>This is a test email to {{Name}} that would be sent to the email address: {{Email}}.</p>";

#endregion

Do the work

#region Do the work

//Loop through the rows
for (int i = 0; i < dt.Rows.Count; i++)

{...}

{
    //Get the data row for this instance
    DataRow row = dt.Rows[i];

    //Create a new body as this'll be updated for each user
    string body = String.Empty;

    //Update the body
    body = emailBody.Replace("##Name##", row["Name"]);
    body = body.Replace("##Email##", row["Email"]);

    litOutput.Text += String.Format("{0}<hr />", body);
}

#endregion

The issue I see with this however is (among others) having 20 fields is a lot to be doing with a find/replace statement as it wouldn’t be very elegant and a nightmare to manage. Sticking with this method of using a dataset I suggested we use a regular expression to match the field delimiters and do a replace that way:

//Create a dataset and add some test columns
DataTable dt = new DataTable();
dt.Columns.Add("Name");
dt.Columns.Add("Email");

Add some test data

#region Add some test data

DataRow dr = dt.NewRow();
dr["Name"] = "Julian";
dr["Email"] = "[email protected]";
dt.Rows.Add(dr);

dr = dt.NewRow();
dr["Name"] = "Tim";
dr["Email"] = "[email protected]";
dt.Rows.Add(dr);

#endregion

Create the example email body

#region Create the example email body

string emailBody = "<p>This is a test email to {{Name}} that would be sent to the email address: {{Email}}.</p>";

#endregion

Do the work

#region Do the work

//Loop through the rows
for (int i = 0; i < dt.Rows.Count; i++)

{...}

{
    //Get the data row for this instance
    DataRow row = dt.Rows[i];

    MatchEvaluator replaceField = delegate(Match m)

    {...};

    {
        return row[m.Groups[1].ToString()].ToString();
    };

    //Create a new body as this'll be updated for each user
    string body = String.Empty;
    //Find the fields
    Regex r = new Regex(@"{{(\w{0,15}?)}}");
    body = r.Replace(emailBody, replaceField);

    litOutput.Text += String.Format("{0}<hr />", body);
}

#endregion

This is alright and in many ways very scaleable. I’m not a fan of DataSets but in this instance it works nicely and does mean expanding the available fields at a later date would just be a matter of adding columns to the query.

How does this relate to accessing a property of an object using a string value instead? Well there was a catch, Julian wasn’t using a DataSet and didn’t want to, he had a collection of custom objects all ready and waiting. As he uses a code generator to generate his Data Access Layer and Business Logic Layer there was a method already exposed allowing you to search for a property by string but it's not a standard .Net method so I decided to work out how it was done.

The solution it turned out was a really rather elegant solution IMHO. Using reflection you can use the same concept as above but with custom objects and Robert is your father’s wife’s sister:

Reflection.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Reflection.aspx.cs" Inherits="Reflection" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h1>Reflection Demo</h1>
        <p>Choose from the following fields to build up your email message, the valid fields are (you can choose whether to use non-valid fields as a test if you like):</p>
        <ul>
            <li>Id</li>
            <li>Email</li>
            <li>Name</li>
            <li>JoinedDate</li>
        </ul>
        <p><asp:CheckBox ID="chkCaseSensitive" runat="server" Text="Make the property search case insensitive" /></p>
        <p><label for="txtEmailBody">Example email body:</label><br />
        <asp:TextBox runat="server" ID="txtEmailBody" TextMode="MultiLine" style="width: 500px; height: 200px;" /></p>
        <p><small>HTML submissions are not allowed and they're encoded anyways so no point in spamming -not that you were going to of course!</small></p>
        <p><asp:Button runat="server" ID="btnSubmit" Text="Merge It!" OnClick="btnSubmit_Click" /></p>
        <asp:Literal ID="litOutput" runat="server" />
    </div>
    </form>
</body>
</html>

Reflection.aspx.cs

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Reflection;

public class TestObject

{...}

{
    private int __Id;
    private string __Name;
    private string __Email;
    private DateTime __JoinedDate;

    public int Id

    {...}

    {
        get

        {...}

        {
            return __Id;
        }

        set

        {...}

        {
            __Id = value;
        }

    }

    public string Name

    {...}

    {
        get

        {...}

        {
            return __Name;
        }

        set

        {...}

        {
            __Name = value;
        }

    }

    public string Email

    {...}

    {
        get

        {...}

        {
            return __Email;
        }

        set

        {...}

        {
            __Email = value;
        }

    }

    public DateTime JoinedDate

    {...}

    {
        get

        {...}

        {
            return __JoinedDate;
        }

        set

        {...}

        {
            __JoinedDate = value;
        }

    }

    public TestObject(int id, string name, string email, DateTime joinedDate)

    {...}

    {
        __Id = id;
        __Name = name;
        __Email = email;
        __JoinedDate = joinedDate;
    }

    public bool GetPropertyValueByName(string propertyName)

    {...}

    {
        object obj = null;
        return this.GetPropertyValueByName(propertyName, false, ref obj);
    }

    public bool GetPropertyValueByName(string propertyName, ref object val)

    {...}

    {
        return this.GetPropertyValueByName(propertyName, false, ref val);
    }

    public bool GetPropertyValueByName(string propertyName, bool caseInsensitive, ref object val)

    {...}

    {
        PropertyInfo p = null;
        BindingFlags flags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;

        //If it's a case-insensitive search then add the flag
        if (caseInsensitive)
            flags = flags | BindingFlags.IgnoreCase;

        p = this.GetType().GetProperty(
               propertyName,
               flags,
               null,
               null,
               Type.EmptyTypes,
               null);

        //Check the property exists and that it has read access
        if (p != null && p.CanRead)

        {...}

        {
            //There is a property that matches the name, we can read it so get it
            val = this.GetType().InvokeMember(
                propertyName,
                BindingFlags.GetProperty | flags,
                null,
                this,
                null);

            //We return true as the user may just want to check that it exists
            return true;
        }

        return false;
    }

}

public partial class Reflection : System.Web.UI.Page

{...}

{
    protected void Page_Load(object sender, EventArgs e)

    {...}

    {
        if (!Page.IsPostBack)

        {...}

        {

            Create the example email body

            #region Create the example email body

            txtEmailBody.Text = "Dear {{Name}},\r\n\r\nThis is a test email that would be sent to the email address: {{Email}}.\r\n\r\n{{Name}} joined on: {{JoinedDate}}. This field should not be found {{Don't Find Me}}\r\n\r\nRegards,\r\n\r\nThe webmaster.";

            #endregion

        }

    }

    protected void btnSubmit_Click(object sender, EventArgs e)

    {...}

    {
        if (Page.IsValid && !String.IsNullOrEmpty(txtEmailBody.Text))

        {...}

        {
            litOutput.Text = "<h2>Output</h2>";

            Perform some basic tests

            #region Perform some basic tests
            litOutput.Text += "<h3>Perform some basic tests:</h3>";
            TestObject testObject = new TestObject(1, "Tim", "[email protected]", DateTime.Today);

            object obj = null;
            if (testObject.GetPropertyValueByName("id", false, ref obj))
                litOutput.Text += String.Format("<li>{0}</li>", obj);
            else
                litOutput.Text += "<li>Doesn't Exist</li>";

            if (testObject.GetPropertyValueByName("name", true, ref obj))
                litOutput.Text += String.Format("<li>{0}</li>", obj);
            else
                litOutput.Text += "<li>Doesn't Exist</li>";

            if (testObject.GetPropertyValueByName("joineddate", true, ref obj))
                litOutput.Text += String.Format("<li>{0}</li>", obj);
            else
                litOutput.Text += "<li>Doesn't Exist</li>";

            if (testObject.GetPropertyValueByName("nothere", true, ref obj))
                litOutput.Text += String.Format("<li>{0}</li>", obj);
            else
                litOutput.Text += "<li>Doesn't Exist</li>";

            #endregion

            Create a collection and add a couple of items

            #region Create a collection and add a couple of items

            List<TestObject> testObjects = new List<TestObject>();
            testObjects.Add(new TestObject(1, "Tim", "[email protected]", DateTime.Parse("01/02/2007")));
            testObjects.Add(new TestObject(2, "Jim", "[email protected]", DateTime.Parse("20/02/2007")));
            testObjects.Add(new TestObject(3, "John", "[email protected]", DateTime.Parse("02/03/2007")));
            testObjects.Add(new TestObject(4, "Gill", "[email protected]", DateTime.Parse("01/04/2007")));
            testObjects.Add(new TestObject(5, "Bill", "[email protected]", DateTime.Parse("11/02/2007")));

            #endregion

            Do the work

            #region Do the work

            //Format it with <pre> for simplicity
            litOutput.Text += "<h3>Now for the reflection example:</h3><hr /><pre>";

            //Loop through the rows
            foreach (TestObject t in testObjects)

            {...}

            {
                MatchEvaluator replaceField = delegate(Match m)

                {...};

                {
                    //Get the property name (depending on your regex but
                    //mine groups the squigly brackets in there incase
                    //a match can't be found
                    string pName = m.Groups[2].ToString();

                    //Check it's not null
                    if (!String.IsNullOrEmpty(pName))

                    {...}

                    {
                        //Create an object that'll be returned from the method
                        object o = null;
                        //Check if that property exists, if it does return it
                        if (t.GetPropertyValueByName(pName, chkCaseSensitive.Checked, ref o))
                            return o.ToString();
                    }

                    //We've not found a match for the property in the object
                    //so return the match instead as it's probably a mistake
                    return m.Value;
                };

                //Create a new body as this'll be updated for each user
                string body = String.Empty;

                //Find the fields within the main body -this can be any of the properties of the object
                Regex r = new Regex(@"({{)(\w{0,15}?)(}})");
                body = r.Replace(txtEmailBody.Text, replaceField);
                //Output the example content (HtmlEncoded so not to hurt us!!)
                litOutput.Text += String.Format("{0}<hr />", Server.HtmlEncode(body));
            }

            litOutput.Text += "</pre>";

            #endregion

        }

    }

}

I’ve thrown up a quick demo if you want to test it out. I think in the longer run I’m going to look into having it generate some form of reporting system as that’d be seriously nice, but the suns out and I need to go for a paddle so that’ll have to wait for another day! So that's my first delve into reflection and so far I love it!