Outputting Errors to User

Question

I currently am making a small application where a user will enter their ID which needs to be in a certain format. When entered in the wrong format, different output errors are produced.

I currently output errors like this:

public void errorLogger(string error)
        {
            errors += error + Environment.NewLine + "<br>";
        }

I have read that using += is not a great operator to use. Alternatively I have thought this may be better:

public void errorLogger(string key, string err)
        {
            if(!String.IsNullOrEmpty(err))
            output.Add(key, err);
        }

but then I basically do the same thing later:

    string keys = "";
    foreach (KeyValuePair<string, string> entry in core.errorLogger())
    {
        keys += entry.Key + ":       " + entry.Value + "<br>";
    }

Is there a cleaner way to output errors? The output is injected under the submit button into:

errorSpan.InnerHtml = (keys);

This is what I ended up using going off of Charles' response to AWinkle

public void logError(string err, string desc)
                {
                    if(!String.IsNullOrEmpty(desc))
                        errors.AppendFormat("{0,10}:&nbsp;{1}<br>", err, desc);
                }
public string getErrors()
            {
                return errors.ToString();
            }

Then :

errorspan.InnerHtml = (core.getErrors());

Answer 1

Using the StringBuilder class will typically be better for looped string building.

string keys = "";
foreach (KeyValuePair<string, string> entry in core.errorLogger())
{
    keys += entry.Key + ":       " + entry.Value + "<br>";
}

turns into

StringBuilder keys = new StringBuilder();
foreach (KeyValuePair<string, string> entry in core.errorLogger())
{
    keys.AppendFormat("{0}:&nbsp;{1}<br>", entry.Key, entry.Value);
}

Then you take errorSpan.InnerHtml = (keys); and change it to errorSpan.InnerHtml = (keys.ToString());

Alternatively, you could use this to replace the dictionary as CharlesNRice suggests.

StringBuilder keys = new StringBuilder();

 public void errorLogger(string key, string err)
    {
        if(!String.IsNullOrEmpty(err))
        keys.AppendFormat("{0}:&nbsp;{1}<br>", key,err);
    }

Answer 2

This answer is another way of achieving the same. I am not claiming that this is the cleanest or most performant way but it certainly works.

List<string>

When you're using this method to store all the error messages, you can use the Enumerable.Aggregate<TSource> method:

var errors = new List<string>();

errors.Add("This is error message 1.");
errors.Add("Another error!");

var message = errors.Aggregate((c, n) => c + Environment.NewLine + n);

Outputted, message will now have following value:

This is error message 1.

Another error!

You can also look into the String.Join method. The message is built up like this:

var message = String.Join(Environment.NewLine, errors);

This will create the same output.

Dictionary<int, string>

In a situation where you store an integer as a key and the message as a string in a Dictionary<int, string>, you can also the String.Join method:

var errors = new Dictionary<int, string>();

errors.Add(1, "This is error message 1.");
errors.Add(2, "Another error!");

var message = String.Join(Environment.NewLine, errors.Select(x => String.Format("{0}: {1}", x.Key, x.Value)));

The result looks like this:

1: This is error message 1.

2: Another error!