diff --git a/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs b/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
index d433fff59b..c7e1bef3b2 100644
--- a/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
+++ b/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
@@ -119,6 +119,7 @@ public static class AbpStringExtensions
     /// <param name="str">The string.</param>
     /// <param name="postFixes">one or more postfix.</param>
     /// <returns>Modified string or the same string if it has not any of given postfixes</returns>
+    [ContractAnnotation("null <= str:null")]
     public static string RemovePostFix(this string str, params string[] postFixes)
     {
         return str.RemovePostFix(StringComparison.Ordinal, postFixes);
@@ -131,6 +132,7 @@ public static class AbpStringExtensions
     /// <param name="comparisonType">String comparison type</param>
     /// <param name="postFixes">one or more postfix.</param>
     /// <returns>Modified string or the same string if it has not any of given postfixes</returns>
+    [ContractAnnotation("null <= str:null")]
     public static string RemovePostFix(this string str, StringComparison comparisonType, params string[] postFixes)
     {
         if (str.IsNullOrEmpty())
@@ -160,6 +162,7 @@ public static class AbpStringExtensions
     /// <param name="str">The string.</param>
     /// <param name="preFixes">one or more prefix.</param>
     /// <returns>Modified string or the same string if it has not any of given prefixes</returns>
+    [ContractAnnotation("null <= str:null")]
     public static string RemovePreFix(this string str, params string[] preFixes)
     {
         return str.RemovePreFix(StringComparison.Ordinal, preFixes);
@@ -172,6 +175,7 @@ public static class AbpStringExtensions
     /// <param name="comparisonType">String comparison type</param>
     /// <param name="preFixes">one or more prefix.</param>
     /// <returns>Modified string or the same string if it has not any of given prefixes</returns>
+    [ContractAnnotation("null <= str:null")]
     public static string RemovePreFix(this string str, StringComparison comparisonType, params string[] preFixes)
     {
         if (str.IsNullOrEmpty())
@@ -264,6 +268,7 @@ public static class AbpStringExtensions
     /// <param name="useCurrentCulture">set true to use current culture. Otherwise, invariant culture will be used.</param>
     /// <param name="handleAbbreviations">set true to if you want to convert 'XYZ' to 'xyz'.</param>
     /// <returns>camelCase of the string</returns>
+    [ContractAnnotation("null <= str:null")]
     public static string ToCamelCase(this string str, bool useCurrentCulture = false, bool handleAbbreviations = false)
     {
         if (string.IsNullOrWhiteSpace(str))
@@ -290,6 +295,7 @@ public static class AbpStringExtensions
     /// </summary>
     /// <param name="str">String to convert.</param>
     /// <param name="useCurrentCulture">set true to use current culture. Otherwise, invariant culture will be used.</param>
+    [ContractAnnotation("null <= str:null")]
     public static string ToSentenceCase(this string str, bool useCurrentCulture = false)
     {
         if (string.IsNullOrWhiteSpace(str))
@@ -307,6 +313,7 @@ public static class AbpStringExtensions
     /// </summary>
     /// <param name="str">String to convert.</param>
     /// <param name="useCurrentCulture">set true to use current culture. Otherwise, invariant culture will be used.</param>
+    [ContractAnnotation("null <= str:null")]
     public static string ToKebabCase(this string str, bool useCurrentCulture = false)
     {
         if (string.IsNullOrWhiteSpace(str))
@@ -440,6 +447,7 @@ public static class AbpStringExtensions
     /// <param name="str">String to convert</param>
     /// <param name="useCurrentCulture">set true to use current culture. Otherwise, invariant culture will be used.</param>
     /// <returns>PascalCase of the string</returns>
+    [ContractAnnotation("null <= str:null")]
     public static string ToPascalCase(this string str, bool useCurrentCulture = false)
     {
         if (string.IsNullOrWhiteSpace(str))
@@ -458,7 +466,7 @@ public static class AbpStringExtensions
     /// <summary>
     /// Gets a substring of a string from beginning of the string if it exceeds maximum length.
     /// </summary>
-    /// <exception cref="ArgumentNullException">Thrown if <paramref name="str"/> is null</exception>
+    [ContractAnnotation("null <= str:null")]
     public static string Truncate(this string str, int maxLength)
     {
         if (str == null)
@@ -477,7 +485,7 @@ public static class AbpStringExtensions
     /// <summary>
     /// Gets a substring of a string from Ending of the string if it exceeds maximum length.
     /// </summary>
-    /// <exception cref="ArgumentNullException">Thrown if <paramref name="str"/> is null</exception>
+    [ContractAnnotation("null <= str:null")]
     public static string TruncateFromBeginning(this string str, int maxLength)
     {
         if (str == null)
@@ -510,6 +518,7 @@ public static class AbpStringExtensions
     /// Returning string can not be longer than maxLength.
     /// </summary>
     /// <exception cref="ArgumentNullException">Thrown if <paramref name="str"/> is null</exception>
+    [ContractAnnotation("null <= str:null")]
     public static string TruncateWithPostfix(this string str, int maxLength, string postfix)
     {
         if (str == null)