Ability to set the DefineConstraints of packages

README.md

@@ -301,6 +301,16 @@ To disable the automatic referencing of assemblies of a NuGet package you can se
 </packages>
 ```
 
+## Define constraints
+To restrict when a NuGet package is referenced based on Scripting Define Symbols, you can set the `defineConstraints` attribute of a package inside the `packages.config`. All defines listed in `defineConstraints` must be present for the package assemblies to be referenced, just like in Unity’s `.asmdef` files. If the attribute is omitted, the package is always referenced.
+_Currently this setting is not available from UI._
+```xml
+<?xml version="1.0" encoding="utf-8" ?>
+<packages>
+    <package id="Serilog" version="2.12.0" autoReferenced="false" defineConstraints="UNITY_EDITOR;DEVELOPMENT_BUILD" />
+</packages>
+```
+
 When this setting is set to `false` the assemblies of the NuGet package are only referenced by Unity projects that explicitly list them inside there `*.asmdef` file.
 
 # How do I create my own NuGet packages from within Unity?

src/NuGetForUnity/Editor/Configuration/PackageConfig.cs

@@ -16,6 +16,15 @@ public class PackageConfig : NugetPackageIdentifier
         /// </summary>
         public bool AutoReferenced { get; set; } = true;
 
+        /// <summary>
+        ///     Gets or sets additional compile-time constraints associated with this package.
+        ///     This property contains a string representing extra compile-time constraints related to the package,
+        ///     such as conditional compilation symbols or conditions for applying the package. Multiple constraints
+        ///     can be combined using a delimiter (a semicolon ';').
+        ///     An empty string means no constraints. The default value is an empty string.
+        /// </summary>
+        public string DefineConstraints { get; set; } = string.Empty;
+
         /// <summary>
         ///     Gets or sets the configured target framework moniker that is used to install this NuGet package instead of
         ///     automatically determining the best matching target framework from the Unity settings ('Api Compatibility Level').

src/NuGetForUnity/Editor/Configuration/PackagesConfigFile.cs

@@ -22,6 +22,8 @@ public class PackagesConfigFile
 
         private const string AutoReferencedAttributeName = "autoReferenced";
 
+        private const string DefineConstraintsAttributeName = "defineConstraints";
+
         private const string TargetFrameworkAttributeName = "targetFramework";
 
         [CanBeNull]
@@ -73,6 +75,7 @@ public static PackagesConfigFile Load()
                         packageElement.Attribute("manuallyInstalled")?.Value.Equals("true", StringComparison.OrdinalIgnoreCase) ?? false,
                     AutoReferenced = (bool)(packageElement.Attributes(AutoReferencedAttributeName).FirstOrDefault() ??
                                             new XAttribute(AutoReferencedAttributeName, true)),
+                    DefineConstraints = packageElement.Attribute(DefineConstraintsAttributeName)?.Value ?? string.Empty,
                     TargetFramework = packageElement.Attribute(TargetFrameworkAttributeName)?.Value,
                 };
                 configFile.Packages.Add(package);
@@ -148,6 +151,11 @@ public void Save()
                     packageElement.Add(new XAttribute(AutoReferencedAttributeName, package.AutoReferenced));
                 }
 
+                if (!string.IsNullOrWhiteSpace(package.DefineConstraints))
+                {
+                    packageElement.Add(new XAttribute(DefineConstraintsAttributeName, package.DefineConstraints));
+                }
+
                 if (!string.IsNullOrEmpty(package.TargetFramework))
                 {
                     packageElement.Add(new XAttribute(TargetFrameworkAttributeName, package.TargetFramework));

src/NuGetForUnity/Editor/NugetAssetPostprocessor.cs

@@ -292,6 +292,11 @@ private static string[] ModifyImportSettingsOfGeneralPlugin([NotNull] PackageCon
         {
             PluginImporterIsExplicitlyReferencedProperty.SetValue(plugin, !packageConfig.AutoReferenced);
 
+            if (!string.IsNullOrWhiteSpace(packageConfig.DefineConstraints))
+            {
+                plugin.DefineConstraints = packageConfig.DefineConstraints.Split(new char[] { ';' }, StringSplitOptions.RemoveEmptyEntries);
+            }
+
             return new[] { ProcessedLabel };
         }