docs: document default values for Filters properties in XML docs

Continues the work started for #5917 (already covered Targets, Layouts,
LayoutRenderers and Target Wrappers), extending the same pattern to the
NLog.Filters namespace so default values show up in IntelliSense and the
generated API docs.

Adds a regression test that scans all settable NLog.Filters properties
against the generated XML documentation file and fails if any of them is
missing a documented default value.

src/NLog/Filters/ConditionBasedFilter.cs

@@ -50,6 +50,7 @@ namespace NLog.Filters
         /// <summary>
         /// Gets or sets the condition expression.
         /// </summary>
+        /// <remarks><b>[Required]</b> Default: <see langword="null"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public ConditionExpression Condition { get; set; } = ConditionLiteralExpression.Null;
 

src/NLog/Filters/Filter.cs

@@ -53,6 +53,7 @@ namespace NLog.Filters
         /// <summary>
         /// Gets or sets the action to be taken when filter matches.
         /// </summary>
+        /// <remarks>Default: <see cref="FilterResult.Neutral"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public FilterResult Action { get; set; }
 

src/NLog/Filters/LayoutBasedFilter.cs

@@ -51,6 +51,7 @@ namespace NLog.Filters
         /// Gets or sets the layout to be used to filter log messages.
         /// </summary>
         /// <value>The layout.</value>
+        /// <remarks><b>[Required]</b> Default: <see cref="Layout.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public Layout Layout { get; set; } = Layout.Empty;
     }

src/NLog/Filters/WhenContainsFilter.cs

@@ -45,12 +45,14 @@ namespace NLog.Filters
         /// <summary>
         /// Gets or sets a value indicating whether to ignore case when comparing strings.
         /// </summary>
+        /// <remarks>Default: <see langword="false"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public bool IgnoreCase { get; set; }
 
         /// <summary>
         /// Gets or sets the substring to be matched.
         /// </summary>
+        /// <remarks><b>[Required]</b> Default: <see cref="string.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public string Substring { get; set; } = string.Empty;
 

src/NLog/Filters/WhenEqualFilter.cs

@@ -45,12 +45,14 @@ namespace NLog.Filters
         /// <summary>
         /// Gets or sets a value indicating whether to ignore case when comparing strings.
         /// </summary>
+        /// <remarks>Default: <see langword="false"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public bool IgnoreCase { get; set; }
 
         /// <summary>
         /// Gets or sets a string to compare the layout to.
         /// </summary>
+        /// <remarks>Default: <see cref="string.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public string CompareTo { get; set; } = string.Empty;
 

src/NLog/Filters/WhenNotContainsFilter.cs

@@ -45,12 +45,14 @@ namespace NLog.Filters
         /// <summary>
         /// Gets or sets the substring to be matched.
         /// </summary>
+        /// <remarks><b>[Required]</b> Default: <see cref="string.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public string Substring { get; set; } = string.Empty;
 
         /// <summary>
         /// Gets or sets a value indicating whether to ignore case when comparing strings.
         /// </summary>
+        /// <remarks>Default: <see langword="false"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public bool IgnoreCase { get; set; }
 

src/NLog/Filters/WhenNotEqualFilter.cs

@@ -52,12 +52,14 @@ namespace NLog.Filters
         /// <summary>
         /// Gets or sets a string to compare the layout to.
         /// </summary>
+        /// <remarks>Default: <see cref="string.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public string CompareTo { get; set; } = string.Empty;
 
         /// <summary>
         /// Gets or sets a value indicating whether to ignore case when comparing strings.
         /// </summary>
+        /// <remarks>Default: <see langword="false"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public bool IgnoreCase { get; set; }
 

src/NLog/Filters/WhenRepeatedFilter.cs

@@ -54,12 +54,14 @@ namespace NLog.Filters
         /// <summary>
         /// How long before a filter expires, and logging is accepted again
         /// </summary>
+        /// <remarks>Default: <c>10</c></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public int TimeoutSeconds { get; set; } = 10;
 
         /// <summary>
         /// Max length of filter values, will truncate if above limit
         /// </summary>
+        /// <remarks>Default: <c>1000</c></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public int MaxLength { get; set; } = 1000;
 
@@ -67,30 +69,35 @@ namespace NLog.Filters
         /// Applies the configured action to the initial logevent that starts the timeout period.
         /// Used to configure that it should ignore all events until timeout.
         /// </summary>
+        /// <remarks>Default: <see langword="false"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public bool IncludeFirst { get; set; }
 
         /// <summary>
         /// Max number of unique filter values to expect simultaneously
         /// </summary>
+        /// <remarks>Default: <c>50000</c></remarks>
         /// <docgen category='Filtering Options' order='100' />
         public int MaxFilterCacheSize { get; set; } = 50000;
 
         /// <summary>
         /// Default number of unique filter values to expect, will automatically increase if needed
         /// </summary>
+        /// <remarks>Default: <c>1000</c></remarks>
         /// <docgen category='Filtering Options' order='100' />
         public int DefaultFilterCacheSize { get; set; } = 1000;
 
         /// <summary>
         /// Insert FilterCount value into <see cref="LogEventInfo.Properties"/> when an event is no longer filtered
         /// </summary>
+        /// <remarks>Default: <see cref="string.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public string FilterCountPropertyName { get; set; } = string.Empty;
 
         /// <summary>
         /// Append FilterCount to the <see cref="LogEventInfo.Message"/> when an event is no longer filtered
         /// </summary>
+        /// <remarks>Default: <see cref="string.Empty"/></remarks>
         /// <docgen category='Filtering Options' order='10' />
         public string FilterCountMessageAppendFormat { get; set; } = string.Empty;
 
@@ -106,6 +113,7 @@ namespace NLog.Filters
         /// <summary>
         /// Default buffer size for the internal buffers
         /// </summary>
+        /// <remarks>Default: <c>1000</c></remarks>
         /// <docgen category='Filtering Options' order='100' />
         public int OptimizeBufferDefaultLength { get; set; } = 1000;
 

tests/NLog.UnitTests/Filters/FilterDefaultValueDocsTests.cs

@@ -0,0 +1,95 @@
+//
+// Copyright (c) 2004-2024 Jaroslaw Kowalski <jaak@jkowalski.net>, Kim Christensen, Julian Verdurmen
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions
+// are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+//
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+//
+// * Neither the name of Jaroslaw Kowalski nor the names of its
+//   contributors may be used to endorse or promote products derived from this
+//   software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+// THE POSSIBILITY OF SUCH DAMAGE.
+//
+
+namespace NLog.UnitTests.Filters
+{
+    using System;
+    using System.Collections.Generic;
+    using System.IO;
+    using System.Linq;
+    using System.Reflection;
+    using System.Xml.Linq;
+    using Xunit;
+
+    /// <summary>
+    /// Verifies that every settable property under the <see cref="NLog.Filters"/> namespace
+    /// documents its default value in the generated XML docs, so it shows up in IntelliSense
+    /// and the Sandcastle API docs. See https://github.com/NLog/NLog/issues/5917
+    /// </summary>
+    public class FilterDefaultValueDocsTests : NLogTestBase
+    {
+        // Properties that intentionally do not document a default value.
+        private static readonly HashSet<string> ExcludedMembers = new HashSet<string>
+        {
+            // Obsolete since NLog 5.0, always returns true, no meaningful default to document.
+            "P:NLog.Filters.WhenRepeatedFilter.OptimizeBufferReuse",
+        };
+
+        [Fact]
+        public void SettablePropertiesDocumentDefaultValue()
+        {
+            var nlogAssembly = typeof(LogManager).Assembly;
+            var xmlDocPath = Path.ChangeExtension(nlogAssembly.Location, ".xml");
+            Assert.True(File.Exists(xmlDocPath), $"Generated XML doc file not found at '{xmlDocPath}'. Build NLog.csproj with GenerateDocumentationFile enabled.");
+
+            var documentedMembers = XDocument.Load(xmlDocPath)
+                .Descendants("member")
+                .ToDictionary(member => (string)member.Attribute("name"), member => member.ToString());
+
+            var propertiesMissingDefaultValueDocs = new List<string>();
+
+            foreach (var type in nlogAssembly.GetTypes())
+            {
+                if (type.Namespace != "NLog.Filters" || !type.IsPublic)
+                    continue;
+
+                foreach (var property in type.GetProperties(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly))
+                {
+                    if (!property.CanWrite || property.GetCustomAttribute<ObsoleteAttribute>() != null)
+                        continue;
+
+                    var memberName = $"P:{type.FullName}.{property.Name}";
+                    if (ExcludedMembers.Contains(memberName))
+                        continue;
+
+                    if (!documentedMembers.TryGetValue(memberName, out var memberDocs) || !memberDocs.Contains("Default:"))
+                    {
+                        propertiesMissingDefaultValueDocs.Add(memberName);
+                    }
+                }
+            }
+
+            Assert.Empty(propertiesMissingDefaultValueDocs);
+        }
+    }
+}