Evaluate standalone variables for truthiness in the preprocessor

Fixes 4770

2 files changed, 48 insertions, 2 deletions

diff --git a/src/wix/WixToolset.Core/Preprocessor.cs b/src/wix/WixToolset.Core/Preprocessor.cs
index 3abe0b5f..084da8d0 100644
--- a/src/wix/WixToolset.Core/Preprocessor.cs
+++ b/src/wix/WixToolset.Core/Preprocessor.cs
@@ -1089,10 +1089,10 @@ namespace WixToolset.Core
             else if (operation.Length == 0)
             {
                 // There is no right side of the equation.
-                // If the variable was evaluated, it exists, so the expression is true
+                // If the variable was evaluated, test to see if it is a "false" value.
                 if (startsWithVariable)
                 {
-                    expressionValue = true;
+                    expressionValue = !(leftValue == String.Empty || leftValue == "0" || leftValue == "false" || leftValue == "no");
                 }
                 else
                 {
diff --git a/src/wix/test/WixToolsetTest.Core/PreprocessorFixture.cs b/src/wix/test/WixToolsetTest.Core/PreprocessorFixture.cs
index b3ad0803..bbae4636 100644
--- a/src/wix/test/WixToolsetTest.Core/PreprocessorFixture.cs
+++ b/src/wix/test/WixToolsetTest.Core/PreprocessorFixture.cs
@@ -61,6 +61,52 @@ namespace WixToolsetTest.Core
             WixAssert.CompareLineByLine(expected, actual);
         }
 
+        [Fact]
+        public void CanPreprocessFalsyCondition()
+        {
+            var input = String.Join(Environment.NewLine,
+                "<Wix>",
+                "<?define A=0?>",
+                "<?if $(A) ?>",
+                "  <Fragment />",
+                "<?endif?>",
+                "</Wix>"
+            );
+            var expected = new[]
+            {
+                "<Wix />"
+            };
+
+            var result = PreprocessFromString(input);
+
+            var actual = result.Document.ToString().Split("\r\n");
+            WixAssert.CompareLineByLine(expected, actual);
+        }
+
+        [Fact]
+        public void CanPreprocessTruthyCondition()
+        {
+            var input = String.Join(Environment.NewLine,
+                "<Wix>",
+                "<?define A=1?>",
+                "<?if $(A) ?>",
+                "  <Fragment />",
+                "<?endif?>",
+                "</Wix>"
+            );
+            var expected = new[]
+            {
+                "<Wix>",
+                "  <Fragment />",
+                "</Wix>"
+            };
+
+            var result = PreprocessFromString(input);
+
+            var actual = result.Document.ToString().Split("\r\n");
+            WixAssert.CompareLineByLine(expected, actual);
+        }
+
         private static IPreprocessResult PreprocessFromString(string xml)
         {
             using var stringReader = new StringReader(xml);