Improve exception message for empty assembly loading

src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

@@ -2018,6 +2018,9 @@
   <data name="BadImageFormat_BadILFormat" xml:space="preserve">
     <value>Bad IL format.</value>
   </data>
+  <data name="BadImageFormat_EmptyAssembly" xml:space="preserve">
+    <value>Assembly image is empty. The stream or byte array must contain a valid PE file.</value>
+  </data>
   <data name="BadImageFormat_InvalidType" xml:space="preserve">
     <value>Corrupt .resources file.  The specified type doesn't exist.</value>
   </data>

src/libraries/System.Private.CoreLib/src/System/Reflection/Assembly.cs

@@ -259,7 +259,7 @@ public static Assembly Load(byte[] rawAssembly, byte[]? rawSymbolStore)
             ArgumentNullException.ThrowIfNull(rawAssembly);
 
             if (rawAssembly.Length == 0)
-                throw new BadImageFormatException(SR.BadImageFormat_BadILFormat);
+                throw new BadImageFormatException(SR.BadImageFormat_EmptyAssembly);
 
             SerializationInfo.ThrowIfDeserializationInProgress("AllowAssembliesFromByteArrays",
                 ref s_cachedSerializationSwitch);

src/libraries/System.Private.CoreLib/src/System/Runtime/Loader/AssemblyLoadContext.cs

@@ -386,7 +386,7 @@ public Assembly LoadFromStream(Stream assembly, Stream? assemblySymbols)
             ReadOnlySpan<byte> spanAssembly = ReadAllBytes(assembly);
             if (spanAssembly.IsEmpty)
             {
-                throw new BadImageFormatException(SR.BadImageFormat_BadILFormat);
+                throw new BadImageFormatException(SR.BadImageFormat_EmptyAssembly);
             }
 
             // Read the symbol stream if provided

src/libraries/System.Runtime/tests/System.Reflection.Tests/AssemblyTests.cs

@@ -841,7 +841,9 @@ public void AssemblyLoadFromBytes()
         public void AssemblyLoadFromBytesNeg()
         {
             Assert.Throws<ArgumentNullException>(() => Assembly.Load((byte[])null));
-            Assert.Throws<BadImageFormatException>(() => Assembly.Load(new byte[0]));
+
+            BadImageFormatException ex = Assert.Throws<BadImageFormatException>(() => Assembly.Load(new byte[0]));
+            Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase);
         }
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsAssemblyLoadingSupported), nameof(PlatformDetection.HasAssemblyFiles))]

src/libraries/System.Runtime/tests/System.Reflection.Tests/CoreCLR/AssemblyTests.cs

@@ -29,6 +29,17 @@ public void LoadFromStream_Location_IsEmpty()
             Assert.Empty(assembly.Location);
         }
 
+        [Fact]
+        public void LoadFromStream_EmptyStream_ThrowsBadImageFormatException()
+        {
+            using (var emptyStream = new MemoryStream())
+            {
+                BadImageFormatException ex = Assert.Throws<BadImageFormatException>(
+                    () => AssemblyLoadContext.Default.LoadFromStream(emptyStream));
+                Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase);
+            }
+        }
+
         [Fact]
         public void EntryPoint()
         {