diff --git a/src/Box2D.NET.Samples/Samples/Collisions/TimeOfImpact.cs b/src/Box2D.NET.Samples/Samples/Collisions/TimeOfImpact.cs
index 344fac6..8d9f0bd 100644
--- a/src/Box2D.NET.Samples/Samples/Collisions/TimeOfImpact.cs
+++ b/src/Box2D.NET.Samples/Samples/Collisions/TimeOfImpact.cs
@@ -66,7 +66,7 @@ public override void Step()
         _input.sweepB = _sweepB;
         _input.maxFraction = 1.0f;
 
-        _output = b2TimeOfImpact(ref _input);
+        _output = b2TimeOfImpact(_input);
     }
 
     public override void Draw()
diff --git a/src/Box2D.NET/B2Distances.cs b/src/Box2D.NET/B2Distances.cs
index dbc1a88..a1c3a25 100644
--- a/src/Box2D.NET/B2Distances.cs
+++ b/src/Box2D.NET/B2Distances.cs
@@ -967,7 +967,7 @@ b2CastOutput b2ShapeCastMerged( const b2ShapeCastPairInput* input, b2ShapeCastDa
         }
 #endif
 
-        public static B2SeparationFunction b2MakeSeparationFunction(ref B2SimplexCache cache, ref B2ShapeProxy proxyA, ref B2Sweep sweepA, ref B2ShapeProxy proxyB, ref B2Sweep sweepB, float t1)
+        public static B2SeparationFunction b2MakeSeparationFunction(ref B2SimplexCache cache, in B2ShapeProxy proxyA, in B2Sweep sweepA, in B2ShapeProxy proxyB, in B2Sweep sweepB, float t1)
         {
             B2SeparationFunction f = new B2SeparationFunction();
 
@@ -1170,7 +1170,7 @@ static float b2EvaluateSeparation(B2SeparationFunction f, int indexA, int indexB
         /// again.
         // CCD via the local separating axis method. This seeks progression
         // by computing the largest time at which separation is maintained.
-        public static B2TOIOutput b2TimeOfImpact(ref B2TOIInput input)
+        public static B2TOIOutput b2TimeOfImpact(in B2TOIInput input)
         {
 #if B2_SNOOP_TOI_COUNTERS
             ulong ticks = b2GetTicks();
@@ -1181,8 +1181,8 @@ public static B2TOIOutput b2TimeOfImpact(ref B2TOIInput input)
             output.state = B2TOIState.b2_toiStateUnknown;
             output.fraction = input.maxFraction;
 
-            B2Sweep sweepA = input.sweepA;
-            B2Sweep sweepB = input.sweepB;
+            ref readonly B2Sweep sweepA = ref input.sweepA;
+            ref readonly B2Sweep sweepB = ref input.sweepB;
             B2_ASSERT(b2IsNormalizedRot(sweepA.q1) && b2IsNormalizedRot(sweepA.q2));
             B2_ASSERT(b2IsNormalizedRot(sweepB.q1) && b2IsNormalizedRot(sweepB.q2));
 
@@ -1190,8 +1190,8 @@ public static B2TOIOutput b2TimeOfImpact(ref B2TOIInput input)
             // c1 can be at the origin yet the points are far away
             // B2Vec2 origin = b2Add(sweepA.c1, input.proxyA.points[0]);
 
-            ref B2ShapeProxy proxyA = ref input.proxyA;
-            ref B2ShapeProxy proxyB = ref input.proxyB;
+            ref readonly B2ShapeProxy proxyA = ref input.proxyA;
+            ref readonly B2ShapeProxy proxyB = ref input.proxyB;
 
             float tMax = input.maxFraction;
 
@@ -1273,7 +1273,7 @@ public static B2TOIOutput b2TimeOfImpact(ref B2TOIInput input)
                 }
 
                 // Initialize the separating axis.
-                B2SeparationFunction fcn = b2MakeSeparationFunction(ref cache, ref proxyA, ref sweepA, ref proxyB, ref sweepB, t1);
+                B2SeparationFunction fcn = b2MakeSeparationFunction(ref cache, proxyA, sweepA, proxyB, sweepB, t1);
 #if FALSE
                     // Dump the curve seen by the root finder
                     {
diff --git a/src/Box2D.NET/B2Solvers.cs b/src/Box2D.NET/B2Solvers.cs
index 63e4247..1756c4b 100644
--- a/src/Box2D.NET/B2Solvers.cs
+++ b/src/Box2D.NET/B2Solvers.cs
@@ -434,7 +434,7 @@ internal static bool b2ContinuousQueryCallback(int proxyId, ulong userData, ref
             input.sweepB = continuousContext.sweep;
             input.maxFraction = continuousContext.fraction;
 
-            B2TOIOutput output = b2TimeOfImpact(ref input);
+            B2TOIOutput output = b2TimeOfImpact(input);
             if (isSensor)
             {
                 // Only accept a sensor hit that is sooner than the current solid hit.
@@ -469,7 +469,7 @@ internal static bool b2ContinuousQueryCallback(int proxyId, ulong userData, ref
                     B2ShapeExtent extent = b2ComputeShapeExtent(fastShape, centroid);
                     float radius = B2_CORE_FRACTION * extent.minExtent;
                     input.proxyB = b2MakeProxy(centroid, 1, radius);
-                    output = b2TimeOfImpact(ref input);
+                    output = b2TimeOfImpact(input);
                     if (0.0f < output.fraction && output.fraction < continuousContext.fraction)
                     {
                         hitFraction = output.fraction;
diff --git a/test/Box2D.NET.Test/B2DistanceTest.cs b/test/Box2D.NET.Test/B2DistanceTest.cs
index cc8c796..82bd12e 100644
--- a/test/Box2D.NET.Test/B2DistanceTest.cs
+++ b/test/Box2D.NET.Test/B2DistanceTest.cs
@@ -113,7 +113,7 @@ public void TimeOfImpactTest()
         input.sweepB = new B2Sweep(b2Vec2_zero, b2Vec2_zero, new B2Vec2(-2.0f, 0.0f), b2Rot_identity, b2Rot_identity);
         input.maxFraction = 1.0f;
 
-        B2TOIOutput output = b2TimeOfImpact(ref input);
+        B2TOIOutput output = b2TimeOfImpact(input);
 
         Assert.That(output.state, Is.EqualTo(B2TOIState.b2_toiStateHit));
         Assert.That(output.fraction - 0.5f, Is.LessThan(0.005f));