Added attribute that can be used to get a description for the value of an exif tag.

Former-commit-id: 3bca9a911b0189867911ef22765009909a9bbe00
Former-commit-id: 91add02607f34749757840aa946b1fe987bd234f
Former-commit-id: cb69236b11426f8c1abd1928c22d0922c1554baa

src/ImageProcessorCore/Profiles/Exif/ExifTag.cs

@@ -159,6 +159,9 @@ namespace ImageProcessorCore
         /// <summary>
         /// ResolutionUnit
         /// </summary>
+        [ExifTagDescription((ushort)1, "None")]
+        [ExifTagDescription((ushort)2, "Inches")]
+        [ExifTagDescription((ushort)3, "Cm")]
         ResolutionUnit = 0x0128,
 
         /// <summary>

src/ImageProcessorCore/Profiles/Exif/ExifTagDescriptionAttribute.cs

@@ -0,0 +1,49 @@
+// <copyright file="ExifTag.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageProcessorCore
+{
+    using System;
+    using System.Linq;
+    using System.Reflection;
+
+    /// <summary>
+    /// Class that provides a description for an ExifTag value.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field, AllowMultiple = true)]
+    internal sealed class ExifTagDescriptionAttribute : Attribute
+    {
+        private object value;
+        private string description;
+
+        ///<summary>
+        /// Initializes a new instance of the ExifTagDescriptionAttribute class.
+        ///</summary>
+        /// <param name="value">The value of the exif tag.</param>
+        /// <param name="description">The description for the value of the exif tag.</param>
+        public ExifTagDescriptionAttribute(object value, string description)
+        {
+            this.value = value;
+            this.description = description;
+        }
+
+        public static string GetDescription(ExifTag tag, object value)
+        {
+            FieldInfo field = tag.GetType().GetTypeInfo().GetDeclaredField(tag.ToString());
+            if (field == null)
+              return null;
+
+            foreach (CustomAttributeData customAttribute in field.CustomAttributes)
+            {
+                object attributeValue = customAttribute.ConstructorArguments[0].Value;
+
+                if (Equals(attributeValue, value))
+                    return (string)customAttribute.ConstructorArguments[1].Value;
+            }
+
+            return null;
+        }
+    }
+}

src/ImageProcessorCore/Profiles/Exif/ExifValue.cs

@@ -561,6 +561,10 @@ namespace ImageProcessorCore
 
         private string ToString(object value)
         {
+            string description = ExifTagDescriptionAttribute.GetDescription(Tag, value);
+            if (description != null)
+              return description;
+
             switch (DataType)
             {
                 case ExifDataType.Ascii:

tests/ImageProcessorCore.Tests/Profiles/Exif/ExifTagDescriptionAttributeTests.cs

@@ -0,0 +1,38 @@
+// <copyright file="ExifProfileTests.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageProcessorCore.Tests
+{
+    using Xunit;
+
+    public class ExifDescriptionAttributeTests
+    {
+        [Fact]
+        public void Test_ExifTag()
+        {
+            var exifProfile = new ExifProfile();
+
+            exifProfile.SetValue(ExifTag.ResolutionUnit, (ushort)1);
+            ExifValue value = exifProfile.GetValue(ExifTag.ResolutionUnit);
+            Assert.Equal("None", value.ToString());
+
+            exifProfile.SetValue(ExifTag.ResolutionUnit, (ushort)2);
+            value = exifProfile.GetValue(ExifTag.ResolutionUnit);
+            Assert.Equal("Inches", value.ToString());
+
+            exifProfile.SetValue(ExifTag.ResolutionUnit, (ushort)3);
+            value = exifProfile.GetValue(ExifTag.ResolutionUnit);
+            Assert.Equal("Cm", value.ToString());
+
+            exifProfile.SetValue(ExifTag.ResolutionUnit, (ushort)4);
+            value = exifProfile.GetValue(ExifTag.ResolutionUnit);
+            Assert.Equal("4", value.ToString());
+
+            exifProfile.SetValue(ExifTag.ImageWidth, 123);
+            value = exifProfile.GetValue(ExifTag.ImageWidth);
+            Assert.Equal("123", value.ToString());
+        }
+    }
+}