NLog auto truncate messages

I don't know of a built in way to do it. Instead, I would write a LayoutRenderer (actually, a WrapperLayoutRenderer). It's not hard.

Something like this (untested) ought to do it:

[LayoutRenderer("truncate")]
[ThreadAgnostic]
public sealed class TruncateLayoutRendererWrapper : WrapperLayoutRendererBase
{
    public TruncateLayoutRendererWrapper()
    {
        this.Truncate = true;
        this.Ellipsis = true;
        this.Limit = 1000;
    }

    [DefaultValue(true)]
    public bool Truncate { get; set; }

    [DefaultValue(true)]
    public bool Ellipsis { get; set; }

    [DefaultValue(1000)]
    public bool Limit { get; set; }

    /// <summary>
    /// Post-processes the rendered message. 
    /// </summary>
    /// <param name="text">The text to be post-processed.</param>
    /// <returns>Trimmed string.</returns>
    protected override string Transform(string text)
    {
        if (!Truncate || Limit <= 0) return text;

        var truncated = text.Substring(0, Ellipsis ? Limit - 3 : Limit);
        if (Ellipsis) truncated += "...";

        return truncated;
    }
}

NLog 4.6.3 supports this:

${message:truncate=1000}

Older versions of NLog can do this:

${trim-whitespace:inner=${message:padding=-1000:fixedLength=true}}

One way to do this is by using regular expression replacement of the message, which you can define right in the nlog.config. I used the following to truncate to 500 characters:

<variable name="truncated_message" value="${replace:replaceWith=...TRUNCATED:regex=true:inner=${message}:searchFor=(?&lt;\=.\{500\}).+}"/>

<target name="filelog" xsi:type="File" fileName="${basedir}/../logs/jobs/${shortdate}.log" layout="${date:format=yyyy-MM-dd HH\:mm\:ss.fff}|${level:uppercase=true}|${truncated_message}"/>