How to log string without surrounded double quotes ?

string m = "message";
logger.Debug("A message: {Message}.", m);

output: A message: "message".

[snakefoot]

You can also do use the :l format parameter (l = literal) to avoid the quotes:

string m = "message";
logger.Debug("A message: {Message:l}.", m);

But yes you can make it application wide by replacing IValueFormatter like this:

        class OverrideValueFormatter : IValueFormatter
        {
            private IValueFormatter _originalFormatter;

            public bool StringWithQuotes { get; set; }

            public OverrideValueFormatter(IValueFormatter originalFormatter)
            {
                _originalFormatter = originalFormatter;
            }

            public bool FormatValue(object value, string format, NLog.MessageTemplates.CaptureType captureType, IFormatProvider formatProvider, System.Text.StringBuilder builder)
            {
                if (!StringWithQuotes && captureType == NLog.MessageTemplates.CaptureType.Normal)
                {
                    switch (Convert.GetTypeCode(value))
                    {
                        case TypeCode.String:
                            {
                                builder.Append((string)value);
                                return true;
                            }
                        case TypeCode.Char:
                            {
                                builder.Append((char)value);
                                return true;
                            }
                        case TypeCode.Empty:
                            return true;  // null becomes empty string
                    }
                }

                return _originalFormatter.FormatValue(value, format, captureType, formatProvider, builder);
            }
        }

And register it like this:

        var oldFormatter = NLog.Config.ConfigurationItemFactory.Default.ValueFormatter;
        var newFormatter = new OverrideValueFormatter(oldFormatter);
        newFormatter.StringWithQuotes = false;
        NLog.Config.ConfigurationItemFactory.Default.ValueFormatter = newFormatter;

[rgbav]

NLog.Config.ConfigurationItemFactory.Default.ValueFormatter is deprecated.

A new way to add the OverrideValueFormatter from @snakefoot would be to define a static method In the Formatter:

public static LoggingConfiguration AssignToConfiguration(LoggingConfiguration configuration)
{
	var originalFormatter = configuration.LogFactory.ServiceRepository.GetService(typeof(IValueFormatter)) as IValueFormatter;
	var newFormatter = new OverrideValueFormatter(originalFormatter)
	{
		StringWithQuotes = false
	};
	configuration.LogFactory.ServiceRepository.RegisterService(typeof(IValueFormatter), newFormatter);

	return configuration;
}

This can than be called f.e. with LogManager.Configuration = OverrideValueFormatter.AssignToConfiguration(LogManager.Configuration);