diff --git a/src/test/data/Filter/MandatoryConstraints.ili b/src/test/data/Filter/MandatoryConstraints.ili
new file mode 100644
index 0000000..546e917
--- /dev/null
+++ b/src/test/data/Filter/MandatoryConstraints.ili
@@ -0,0 +1,30 @@
+INTERLIS 2.4;
+
+MODEL TestSuite
+  AT "mailto:info@geowerkstatt.ch" VERSION "2024-04-04" =
+  IMPORTS GeoW_FunctionsExt;
+
+  TOPIC FunctionTestTopic =
+
+    STRUCTURE ReferencedStruct =
+      textAttr: TEXT*16;
+      enumAttr: (val1,val2,val3);
+      numberAttr: 0..10;
+    END ReferencedStruct;
+
+    STRUCTURE ExtendedStruct EXTENDS ReferencedStruct =
+      newAttr: BOOLEAN;
+    END ExtendedStruct;
+
+    CLASS BaseClass =
+      references: BAG {1..*} OF ReferencedStruct;
+
+      MANDATORY CONSTRAINT trueConstraintEnumAttr: INTERLIS.elementCount(GeoW_FunctionsExt.Filter(THIS->references, "WHERE enumAttr == #val2;")) == 3;
+      MANDATORY CONSTRAINT trueConstraintNumberAttr: INTERLIS.elementCount(GeoW_FunctionsExt.Filter(references, "WHERE numberAttr >= 3 AND enumAttr != #val3;")) == 1;
+      MANDATORY CONSTRAINT falseConstraintEnumAttr: INTERLIS.elementCount(GeoW_FunctionsExt.Filter(references, "WHERE enumAttr == #val2;")) == 0;
+      MANDATORY CONSTRAINT falseConstraintNumberAttr: INTERLIS.elementCount(GeoW_FunctionsExt.Filter(THIS->references, "WHERE numberAttr == 3 AND enumAttr == #val3;")) > 0;
+    END BaseClass;
+
+  END FunctionTestTopic;
+
+END TestSuite.
diff --git a/src/test/data/Filter/MandatoryConstraintsText.ili b/src/test/data/Filter/MandatoryConstraintsText.ili
new file mode 100644
index 0000000..e0e57cb
--- /dev/null
+++ b/src/test/data/Filter/MandatoryConstraintsText.ili
@@ -0,0 +1,28 @@
+INTERLIS 2.4;
+
+MODEL TestSuite
+  AT "mailto:info@geowerkstatt.ch" VERSION "2024-04-04" =
+  IMPORTS GeoW_FunctionsExt;
+
+  TOPIC FunctionTestTopic =
+
+    STRUCTURE ReferencedStruct =
+      textAttr: TEXT*16;
+      enumAttr: (val1,val2,val3);
+      numberAttr: 0..10;
+    END ReferencedStruct;
+
+    STRUCTURE ExtendedStruct EXTENDS ReferencedStruct =
+      newAttr: BOOLEAN;
+    END ExtendedStruct;
+
+    CLASS BaseClass =
+      references: BAG {1..*} OF ReferencedStruct;
+
+      MANDATORY CONSTRAINT trueConstraintTextAttr: INTERLIS.elementCount(GeoW_FunctionsExt.Filter(THIS->references, "WHERE textAttr == \"Some Value\";")) == 2;
+      MANDATORY CONSTRAINT falseConstraintTextAttr: INTERLIS.elementCount(GeoW_FunctionsExt.Filter(references, "WHERE textAttr == \"Value that does not exist\";")) > 0;
+    END BaseClass;
+
+  END FunctionTestTopic;
+
+END TestSuite.
diff --git a/src/test/data/Filter/TestData.xtf b/src/test/data/Filter/TestData.xtf
new file mode 100644
index 0000000..db84c90
--- /dev/null
+++ b/src/test/data/Filter/TestData.xtf
@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ili:transfer xmlns:ili="http://www.interlis.ch/xtf/2.4/INTERLIS" xmlns:geom="http://www.interlis.ch/geometry/1.0"
+              xmlns:TestSuite="http://www.interlis.ch/xtf/2.4/TestSuite">
+    <ili:headersection>
+        <ili:models>
+            <ili:model>GeoW_FunctionsExt</ili:model>
+            <ili:model>TestSuite</ili:model>
+        </ili:models>
+        <ili:sender>ili2gpkg-4.6.1-63db90def1260a503f0f2d4cb846686cd4851184</ili:sender>
+    </ili:headersection>
+    <ili:datasection>
+        <TestSuite:FunctionTestTopic ili:bid="TestSuite.FunctionTestTopic">
+            <TestSuite:BaseClass ili:tid="base">
+                <TestSuite:references>
+                    <TestSuite:ReferencedStruct>
+                        <TestSuite:textAttr>Some Value</TestSuite:textAttr>
+                        <TestSuite:enumAttr>val2</TestSuite:enumAttr>
+                        <TestSuite:numberAttr>2</TestSuite:numberAttr>
+                    </TestSuite:ReferencedStruct>
+                    <TestSuite:ReferencedStruct>
+                        <TestSuite:textAttr>aaa</TestSuite:textAttr>
+                        <TestSuite:enumAttr>val2</TestSuite:enumAttr>
+                        <TestSuite:numberAttr>2</TestSuite:numberAttr>
+                    </TestSuite:ReferencedStruct>
+                    <TestSuite:ReferencedStruct>
+                        <TestSuite:textAttr>aaa</TestSuite:textAttr>
+                        <TestSuite:enumAttr>val2</TestSuite:enumAttr>
+                        <TestSuite:numberAttr>2</TestSuite:numberAttr>
+                    </TestSuite:ReferencedStruct>
+                    <TestSuite:ExtendedStruct>
+                        <TestSuite:textAttr>Some Value</TestSuite:textAttr>
+                        <TestSuite:enumAttr>val3</TestSuite:enumAttr>
+                        <TestSuite:numberAttr>1</TestSuite:numberAttr>
+                        <TestSuite:newAttr>false</TestSuite:newAttr>
+                    </TestSuite:ExtendedStruct>
+                    <TestSuite:ReferencedStruct>
+                        <TestSuite:textAttr>bbb</TestSuite:textAttr>
+                        <TestSuite:enumAttr>val1</TestSuite:enumAttr>
+                        <TestSuite:numberAttr>3</TestSuite:numberAttr>
+                    </TestSuite:ReferencedStruct>
+                </TestSuite:references>
+            </TestSuite:BaseClass>
+        </TestSuite:FunctionTestTopic>
+    </ili:datasection>
+</ili:transfer>
diff --git a/src/test/java/ch/geowerkstatt/ilivalidator/extensions/functions/FilterIoxPluginTest.java b/src/test/java/ch/geowerkstatt/ilivalidator/extensions/functions/FilterIoxPluginTest.java
new file mode 100644
index 0000000..498015d
--- /dev/null
+++ b/src/test/java/ch/geowerkstatt/ilivalidator/extensions/functions/FilterIoxPluginTest.java
@@ -0,0 +1,38 @@
+package ch.geowerkstatt.ilivalidator.extensions.functions;
+
+import ch.interlis.ili2c.Ili2cFailure;
+import ch.interlis.iox.IoxException;
+import com.vividsolutions.jts.util.Assert;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+class FilterIoxPluginTest {
+    protected static final String TEST_DATA = "Filter/TestData.xtf";
+    private ValidationTestHelper vh;
+
+    @BeforeEach
+    public void setUp() {
+        vh = new ValidationTestHelper();
+        vh.addFunction(new FilterIoxPlugin());
+    }
+
+    @Test
+    public void mandatoryConstraint() throws Ili2cFailure, IoxException {
+        vh.runValidation(new String[]{TEST_DATA}, new String[]{"Filter/MandatoryConstraints.ili"});
+        Assert.equals(2, vh.getErrs().size());
+        AssertionHelper.assertNoConstraintError(vh, "trueConstraintEnumAttr");
+        AssertionHelper.assertNoConstraintError(vh, "trueConstraintNumberAttr");
+        AssertionHelper.assertConstraintErrors(vh, 1, "base", "falseConstraintEnumAttr");
+        AssertionHelper.assertConstraintErrors(vh, 1, "base", "falseConstraintNumberAttr");
+    }
+
+    @Test
+    @Disabled("Escape sequences in strings (https://github.com/claeis/ili2c/issues/124)")
+    public void filterTextAttr() throws Ili2cFailure, IoxException {
+        vh.runValidation(new String[]{TEST_DATA}, new String[]{"Filter/MandatoryConstraintsText.ili"});
+        Assert.equals(1, vh.getErrs().size());
+        AssertionHelper.assertNoConstraintError(vh, "trueConstraintTextAttr");
+        AssertionHelper.assertConstraintErrors(vh, 1, "base", "falseConstraintTextAttr");
+    }
+}