diff --git a/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs b/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
index 0e71df6b59..c7e1bef3b2 100644
--- a/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
+++ b/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
@@ -46,6 +46,7 @@ public static class AbpStringExtensions
     /// <summary>
     /// Indicates whether this string is null or an System.String.Empty string.
     /// </summary>
+    [ContractAnnotation("str:null => true")]
     public static bool IsNullOrEmpty(this string str)
     {
         return string.IsNullOrEmpty(str);
@@ -54,6 +55,7 @@ public static class AbpStringExtensions
     /// <summary>
     /// indicates whether this string is null, empty, or consists only of white-space characters.
     /// </summary>
+    [ContractAnnotation("str:null => true")]
     public static bool IsNullOrWhiteSpace(this string str)
     {
         return string.IsNullOrWhiteSpace(str);
@@ -117,6 +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);
@@ -129,6 +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())
@@ -158,6 +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);
@@ -170,6 +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())
@@ -262,6 +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))
@@ -288,6 +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))
@@ -305,6 +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))
@@ -438,6 +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))
@@ -456,7 +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)
@@ -475,7 +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)
@@ -508,6 +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)
diff --git a/framework/src/Volo.Abp.Core/System/Collections/Generic/AbpCollectionExtensions.cs b/framework/src/Volo.Abp.Core/System/Collections/Generic/AbpCollectionExtensions.cs
index 4df3010aba..83fbf59629 100644
--- a/framework/src/Volo.Abp.Core/System/Collections/Generic/AbpCollectionExtensions.cs
+++ b/framework/src/Volo.Abp.Core/System/Collections/Generic/AbpCollectionExtensions.cs
@@ -12,6 +12,7 @@ public static class AbpCollectionExtensions
     /// <summary>
     /// Checks whatever given collection object is null or has no item.
     /// </summary>
+    [ContractAnnotation("source:null => true")]
     public static bool IsNullOrEmpty<T>([CanBeNull] this ICollection<T> source)
     {
         return source == null || source.Count <= 0;