diff --git a/src/SwaggerProvider.Runtime/RuntimeHelpers.fs b/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
index 9f2390c..340ae49 100644
--- a/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
+++ b/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
@@ -312,8 +312,10 @@ module RuntimeHelpers =
             | :? IO.Stream as stream -> addFileStream name stream
             | :? (IO.Stream[]) as streams -> streams |> Seq.iter(addFileStream name)
             | x ->
-                let strValue = x.ToString() // TODO: serialize? does not work with arrays probably
-                cnt.Add(toStringContent strValue, name)
+                let strValue = toParam x
+
+                if not(isNull strValue) then
+                    cnt.Add(toStringContent strValue, name)
 
         cnt
 
@@ -321,7 +323,13 @@ module RuntimeHelpers =
         let keyValues =
             keyValues
             |> Seq.filter(snd >> isNull >> not)
-            |> Seq.map(fun (k, v) -> Collections.Generic.KeyValuePair(k, v.ToString()))
+            |> Seq.choose(fun (k, v) ->
+                let param = toParam v
+
+                if isNull param then
+                    None
+                else
+                    Some(Collections.Generic.KeyValuePair(k, param)))
 
         new FormUrlEncodedContent(keyValues)
 
diff --git a/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs b/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
index e2d0b10..7adb271 100644
--- a/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
+++ b/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
@@ -738,6 +738,65 @@ module ToFormUrlEncodedContentTests =
             body |> shouldEqual ""
         }
 
+    [<Fact>]
+    let ``toFormUrlEncodedContent formats DateTime as ISO 8601``() =
+        task {
+            let dt = DateTime(2024, 6, 15, 10, 30, 0, DateTimeKind.Utc)
+
+            use content = toFormUrlEncodedContent(seq { ("ts", box dt) })
+
+            let! body = content.ReadAsStringAsync()
+            let encodedValue = body.Substring("ts=".Length)
+            let decodedValue = WebUtility.UrlDecode(encodedValue)
+
+            decodedValue |> shouldEqual(dt.ToString("O"))
+        }
+
+    [<Fact>]
+    let ``toFormUrlEncodedContent formats DateTimeOffset as ISO 8601``() =
+        task {
+            let dto = DateTimeOffset(2024, 6, 15, 10, 30, 0, TimeSpan.Zero)
+
+            use content = toFormUrlEncodedContent(seq { ("ts", box dto) })
+
+            let! body = content.ReadAsStringAsync()
+            let encodedValue = body.Substring("ts=".Length)
+            let decodedValue = WebUtility.UrlDecode(encodedValue)
+
+            decodedValue |> shouldEqual(dto.ToString("O"))
+        }
+
+    [<Fact>]
+    let ``toFormUrlEncodedContent formats DateOnly as ISO 8601``() =
+        task {
+            let d = DateOnly(2025, 7, 4)
+            use content = toFormUrlEncodedContent(seq { ("date", box d) })
+
+            let! body = content.ReadAsStringAsync()
+            let encodedValue = body.Substring("date=".Length)
+            let decodedValue = WebUtility.UrlDecode(encodedValue)
+
+            decodedValue |> shouldEqual "2025-07-04"
+        }
+
+    [<Fact>]
+    let ``toFormUrlEncodedContent skips values when toParam returns null``() =
+        task {
+            let nestedNone = box(Some(None: string option))
+
+            use content =
+                toFormUrlEncodedContent(
+                    seq {
+                        ("present", box "yes")
+                        ("nestedNone", nestedNone)
+                    }
+                )
+
+            let! body = content.ReadAsStringAsync()
+            body |> shouldContainText "present=yes"
+            body |> shouldNotContainText "nestedNone"
+        }
+
 
 module ToMultipartFormDataContentTests =
 
@@ -778,6 +837,59 @@ module ToMultipartFormDataContentTests =
 
         hasFileName |> shouldEqual true
 
+    [<Fact>]
+    let ``toMultipartFormDataContent formats DateTime as ISO 8601``() =
+        task {
+            let dt = DateTime(2024, 6, 15, 10, 30, 0, DateTimeKind.Utc)
+            use content = toMultipartFormDataContent(seq { ("ts", box dt) })
+            let part = content |> Seq.exactlyOne
+            let! body = part.ReadAsStringAsync()
+            body |> shouldEqual(dt.ToString("O"))
+        }
+
+    [<Fact>]
+    let ``toMultipartFormDataContent formats DateTimeOffset as ISO 8601``() =
+        task {
+            let dto = DateTimeOffset(2024, 6, 15, 10, 30, 0, TimeSpan.Zero)
+            use content = toMultipartFormDataContent(seq { ("ts", box dto) })
+            let part = content |> Seq.exactlyOne
+            let! body = part.ReadAsStringAsync()
+            body |> shouldEqual(dto.ToString("O"))
+        }
+
+    [<Fact>]
+    let ``toMultipartFormDataContent formats DateOnly as ISO 8601``() =
+        task {
+            let d = DateOnly(2025, 7, 4)
+            use content = toMultipartFormDataContent(seq { ("date", box d) })
+            let part = content |> Seq.exactlyOne
+            let! body = part.ReadAsStringAsync()
+            body |> shouldEqual "2025-07-04"
+        }
+
+    [<Fact>]
+    let ``toMultipartFormDataContent skips values when toParam returns null``() =
+        task {
+            let nestedNone = box(Some(None: string option))
+
+            use content =
+                toMultipartFormDataContent(
+                    seq {
+                        ("present", box "yes")
+                        ("nestedNone", nestedNone)
+                    }
+                )
+
+            content |> Seq.length |> shouldEqual 1
+            let part = content |> Seq.exactlyOne
+            let! body = part.ReadAsStringAsync()
+
+            part.Headers.ContentDisposition.Name.Trim('"')
+            |> shouldEqual "present"
+
+            body |> shouldEqual "yes"
+        }
+
 
 /// Test types for getPropertyValues tests.
 type PropValWithAttr(value: string) =