Added option to throw an exception for unresolved variables.

ConfigurationSubstitution.Tests/ConfigurationTests.cs

@@ -121,5 +121,39 @@ public void Should_get_non_substituted_value_as_is()
 
             substituted.Should().Be("Boyz n the hood");
         }
+
+        [Fact]
+        public void Should_throw_for_non_resolved_variable()
+        {
+            var configurationBuilder = new ConfigurationBuilder()
+                .AddInMemoryCollection(new Dictionary<string, string>
+                {
+                    { "TestKey", "Test value {Foobar}" }
+                })
+                .EnableSubstitutions();
+
+            var configuration = configurationBuilder.Build();
+
+            // Act
+            Action act = () => _ = configuration["TestKey"];
+
+            act.Should().Throw<UndefinedConfigVariableException>().WithMessage("*variable*{Foobar}*");
+        }
+
+        [Fact]
+        public void Should_ignore_non_resolved_variable()
+        {
+            var configurationBuilder = new ConfigurationBuilder()
+                .AddInMemoryCollection(new Dictionary<string, string>
+                {
+                    { "TestKey", "Test value {Foobar}" }
+                })
+                .EnableSubstitutions(exceptionOnMissingVariables: false);
+
+            var configuration = configurationBuilder.Build();
+
+            var value = configuration["TestKey"];
+            value.Should().Be("Test value ");
+        }
     }
 }

ConfigurationSubstitutor/ConfigurationSubstitution.csproj

@@ -18,7 +18,7 @@
     <RepositoryType>GitHub</RepositoryType>
     <PackageProjectUrl>https://github.com/molinch/ConfigurationSubstitutor</PackageProjectUrl>
     <PackageTags>netcore options configuration substitute substitution substituted variable aspnet</PackageTags>
-    <Version>1.0.2</Version>
+    <Version>1.1.0</Version>
   </PropertyGroup>
 
   <ItemGroup>

ConfigurationSubstitutor/ConfigurationSubstitutor.cs

@@ -9,16 +9,18 @@ public class ConfigurationSubstitutor
         private readonly string _startsWith;
         private readonly string _endsWith;
         private Regex _findSubstitutions;
+        private readonly bool _exceptionOnMissingVariables;
 
-        public ConfigurationSubstitutor() : this("{", "}")
+        public ConfigurationSubstitutor(bool exceptionOnMissingVariables = true) : this("{", "}", exceptionOnMissingVariables)
         {
         }
 
-        public ConfigurationSubstitutor(string substitutableStartsWith, string substitutableEndsWith)
+        public ConfigurationSubstitutor(string substitutableStartsWith, string substitutableEndsWith, bool exceptionOnMissingVariables = true)
         {
             _startsWith = substitutableStartsWith;
             _endsWith = substitutableEndsWith;
             _findSubstitutions = new Regex(@"(?<=" + Regex.Escape(_startsWith) + @")[^}{]*(?="+ Regex.Escape(_endsWith) + @")", RegexOptions.Compiled);
+            _exceptionOnMissingVariables = exceptionOnMissingVariables;
         }
 
         public string GetSubstituted(IConfiguration configuration, string key)
@@ -34,7 +36,14 @@ public string ApplySubstitution(IConfiguration configuration, string value)
             var captures = _findSubstitutions.Matches(value).Cast<Match>().SelectMany(m => m.Captures.Cast<Capture>());
             foreach (var capture in captures)
             {
-                value = value.Replace(_startsWith + capture.Value + _endsWith, configuration[capture.Value]);
+                var substitutedValue = configuration[capture.Value];
+
+                if (substitutedValue == null && _exceptionOnMissingVariables)
+                {
+                    throw new UndefinedConfigVariableException($"{_startsWith}{capture.Value}{_endsWith}");
+                }
+
+                value = value.Replace(_startsWith + capture.Value + _endsWith, substitutedValue);
             }
             return value;
         }

ConfigurationSubstitutor/IConfigurationBuilderExtensions.cs

@@ -4,14 +4,14 @@ namespace ConfigurationSubstitution
 {
     public static class IConfigurationBuilderExtensions
     {
-        public static IConfigurationBuilder EnableSubstitutions(this IConfigurationBuilder builder)
+        public static IConfigurationBuilder EnableSubstitutions(this IConfigurationBuilder builder, bool exceptionOnMissingVariables = true)
         {
-            return EnableSubstitutions(builder, new ConfigurationSubstitutor());
+            return EnableSubstitutions(builder, new ConfigurationSubstitutor(exceptionOnMissingVariables));
         }
 
-        public static IConfigurationBuilder EnableSubstitutions(this IConfigurationBuilder builder, string substitutableStartsWith, string substitutableEndsWith)
+        public static IConfigurationBuilder EnableSubstitutions(this IConfigurationBuilder builder, string substitutableStartsWith, string substitutableEndsWith, bool exceptionOnMissingVariables = true)
         {
-            return EnableSubstitutions(builder, new ConfigurationSubstitutor(substitutableStartsWith, substitutableEndsWith));
+            return EnableSubstitutions(builder, new ConfigurationSubstitutor(substitutableStartsWith, substitutableEndsWith, exceptionOnMissingVariables));
         }
 
         private static IConfigurationBuilder EnableSubstitutions(this IConfigurationBuilder builder, ConfigurationSubstitutor substitutor)

ConfigurationSubstitutor/UndefinedConfigVariableException.cs

@@ -0,0 +1,12 @@
+namespace ConfigurationSubstitution
+{
+    using System;
+
+    public class UndefinedConfigVariableException : Exception
+    {
+        public UndefinedConfigVariableException(string variableName)
+            : base($"No value found for configuration variable {variableName}")
+        {
+        }
+    }
+}