diff --git a/CodeConverter/CSharp/CommonConversions.cs b/CodeConverter/CSharp/CommonConversions.cs
index 6647937f..8af2c712 100644
--- a/CodeConverter/CSharp/CommonConversions.cs
+++ b/CodeConverter/CSharp/CommonConversions.cs
@@ -664,11 +664,28 @@ public bool HasOutAttribute(VBSyntax.AttributeListSyntax a)
 
     public bool IsExtensionAttribute(VBSyntax.AttributeSyntax a)
     {
+        if (a.SyntaxTree != SemanticModel.SyntaxTree)
+            return AttributeNameMatches(a, "Extension");
         return (SemanticModel.GetTypeInfo(a).ConvertedType?.GetFullMetadataName())
             ?.Equals(ExtensionAttributeType.FullName, StringComparison.Ordinal) == true;
     }
 
-    public bool IsOutAttribute(VBSyntax.AttributeSyntax a) => SemanticModel.GetTypeInfo(a).ConvertedType.IsOutAttribute();
+    public bool IsOutAttribute(VBSyntax.AttributeSyntax a)
+    {
+        if (a.SyntaxTree != SemanticModel.SyntaxTree)
+            return AttributeNameMatches(a, "Out");
+        return SemanticModel.GetTypeInfo(a).ConvertedType.IsOutAttribute();
+    }
+
+    // SemanticModel.GetTypeInfo throws when the node is not in its syntax tree; fall back to name matching.
+    private static bool AttributeNameMatches(VBSyntax.AttributeSyntax a, string shortName)
+    {
+        var name = a.Name.ToString();
+        return name.Equals(shortName, StringComparison.Ordinal) ||
+               name.Equals(shortName + "Attribute", StringComparison.Ordinal) ||
+               name.EndsWith("." + shortName, StringComparison.Ordinal) ||
+               name.EndsWith("." + shortName + "Attribute", StringComparison.Ordinal);
+    }
 
     public ISymbol GetCsOriginalSymbolOrNull(ISymbol symbol)
     {
diff --git a/Tests/CSharp/ExpressionTests/ByRefTests.cs b/Tests/CSharp/ExpressionTests/ByRefTests.cs
index a5badfd3..2d0be02d 100644
--- a/Tests/CSharp/ExpressionTests/ByRefTests.cs
+++ b/Tests/CSharp/ExpressionTests/ByRefTests.cs
@@ -1,4 +1,6 @@
 using System.Threading.Tasks;
+using ICSharpCode.CodeConverter.Common;
+using ICSharpCode.CodeConverter.CSharp;
 using ICSharpCode.CodeConverter.Tests.TestRunners;
 using Xunit;
 
@@ -914,4 +916,49 @@ public static void LogAndReset(ref int arg)
 }");
     }
 
+    [Fact]
+    public async Task Issue1225_OutAttributeOnParameterFromOtherFileDoesNotCrashAsync()
+    {
+        // Issue #1225: IsOutAttribute called SemanticModel.GetTypeInfo(attribute) on an attribute node
+        // that belongs to a different syntax tree (parameter declared in a separate source file).
+        // This caused ArgumentException "Knoten ist nicht innerhalb Syntaxbaum" /
+        // "Node is not within syntax tree". The fix falls back to name-based checking
+        // when the attribute's syntax tree differs from the current semantic model's tree.
+        var serviceFileContent = @"Imports System.Runtime.InteropServices
+Public Class LicenseService
+    Public Shared ReadOnly Property Instance As LicenseService
+        Get
+            Return New LicenseService()
+        End Get
+    End Property
+
+    Public Function GetLicenseMaybe(<Out> ByRef licenseName As String) As Boolean
+        licenseName = Nothing
+        Return False
+    End Function
+End Class";
+
+        var callerFileContent = @"Public Class Caller
+    Public Sub Test(licenseName As String)
+        Dim res = LicenseService.Instance.GetLicenseMaybe(licenseName)
+    End Sub
+End Class";
+
+        var options = new TextConversionOptions(DefaultReferences.With()) { ShowCompilationErrors = true };
+        var languageConversion = new VBToCSConversion { ConversionOptions = options };
+        var serviceTree = languageConversion.CreateTree(serviceFileContent);
+        var callerTree = languageConversion.CreateTree(callerFileContent);
+
+        // Add both files to the same project so that the VB compilation sees both
+        var serviceDoc = await languageConversion.CreateProjectDocumentFromTreeAsync(serviceTree, options.References);
+        var callerDoc = serviceDoc.Project.AddDocumentFromTree(callerTree);
+
+        // Convert only the caller document; its parameter info comes from the service file's syntax tree
+        var result = await ProjectConversion.ConvertSingleAsync<VBToCSConversion>(callerDoc, options);
+        var output = (result.ConvertedCode ?? "") + (result.GetExceptionsAsString() ?? "");
+
+        Assert.DoesNotContain("#error", output);
+        Assert.Contains("GetLicenseMaybe", output);
+    }
+
 }
\ No newline at end of file