ConditionBasedFilter.cs

// 
// Copyright (c) 2004-2019 Jaroslaw Kowalski <jaak@jkowalski.net>, Kim Christensen, Julian Verdurmen
// 
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namespace NLog.Filters
{
    using Conditions;
    using Config;

    /// <summary>
    /// Matches when the specified condition is met.
    /// </summary>
    /// <remarks>
    /// Conditions are expressed using a simple language 
    /// described <a href="conditions.html">here</a>.
    /// </remarks>
    [Filter("when")]
    public class ConditionBasedFilter : Filter
    {
        private static readonly object boxedTrue = true;

        /// <summary>
        /// Gets or sets the condition expression.
        /// </summary>
        /// <docgen category='Filtering Options' order='10' />
        [RequiredParameter]
        public ConditionExpression Condition { get; set; }

        internal FilterResult DefaultFilterResult { get; set; } = FilterResult.Neutral;

        /// <summary>
        /// Checks whether log event should be logged or not.
        /// </summary>
        /// <param name="logEvent">Log event.</param>
        /// <returns>
        /// <see cref="FilterResult.Ignore"/> - if the log event should be ignored<br/>
        /// <see cref="FilterResult.Neutral"/> - if the filter doesn't want to decide<br/>
        /// <see cref="FilterResult.Log"/> - if the log event should be logged<br/>
        /// .</returns>
        protected override FilterResult Check(LogEventInfo logEvent)
        {
            object val = Condition.Evaluate(logEvent);
            if (boxedTrue.Equals(val))
            {
                return Action;
            }

            return DefaultFilterResult;
        }
    }
}