使用 roslyn 获取属性参数

Question

I try to get the named arguments for MyAttribute with Roslyn.

var sourceCode = (@"
    public class MyAttribute : Attribute
    {
        public string Test { get; set; }
    }

    [MyAttribute(Test = ""Hello"")]
    public class MyClass { }
");

var syntaxTree = CSharpSyntaxTree.ParseText(sourceCode);
var mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
var compilation = CSharpCompilation.Create("MyCompilation", new[] { syntaxTree }, new[] { mscorlib });
var semanticModel = compilation.GetSemanticModel(syntaxTree);

var syntaxRoot = syntaxTree.GetRoot();
var classNode = syntaxRoot.DescendantNodes().OfType<ClassDeclarationSyntax>().Skip(1).First();
var classModel = (ITypeSymbol)semanticModel.GetDeclaredSymbol(classNode);
var firstAttribute = classModel.GetAttributes().First();

However firstAttribute.AttributeClass.Kind equals to ErrorType and consequently firstAttribute.NamedArguments contains no elements.

The code isn't an anlyzer or something I have more complete context like a solution.

I can't see roslyn is missing any references or something else. What can I do to fully analyze the attribute?

Answer 1

You need to fully qualify Attribute type name:

var sourceCode = (@"
    public class MyAttribute : System.Attribute // < here
    {
        public string Test { get; set; }
    }

    [MyAttribute(Test = ""Hello"")]
    public class MyClass { }
");

Then it will work as you expect:

var firstNamedArg = firstAttribute.NamedArguments[0];
var key = firstNamedArg.Key; // "Test"
var value = firstNamedArg.Value.Value; // "Hello"

Alternatively, you can add using System; at the top:

var sourceCode = (@"
    using System;
    public class MyAttribute : Attribute
    {
        public string Test { get; set; }
    }

    [MyAttribute(Test = ""Hello"")]
    public class MyClass { }
");

Answer 2

Instead of using the Roslyn's SemanticModel you can also simply use the Syntax API to get the atttribute's argument info:

        var firstAttribute = classNode.AttributeLists.First().Attributes.First();
        var attributeName = firstAttribute.Name.NormalizeWhitespace().ToFullString();
        Console.WriteLine(attributeName);
        // prints --> "MyAttribute"

        var firstArgument = firstAttribute.ArgumentList.Arguments.First();

        var argumentFullString = firstArgument.NormalizeWhitespace().ToFullString();
        Console.WriteLine(argumentFullString);
        // prints --> Test = "Hello"

        var argumentName = firstArgument.NameEquals.Name.Identifier.ValueText;
        Console.WriteLine(argumentName);
        // prints --> Test

        var argumentExpression = firstArgument.Expression.NormalizeWhitespace().ToFullString();
        Console.WriteLine(argumentExpression);
        // prints --> "Hello"

Answer 3

Once you have your xxxDeclaredSymbol you can get all attributes like so

var attributes = methodSymbol.GetAttributes().ToArray();

You can then loop over each AttributeData in the array and use this extension I wrote, which will give you a list of all names + values passed to the attribute's constructor whether they were named or not...

// Used as a 3 value tuple for Name + TypeName + actual value
public class NameTypeAndValue
{
    public string Name { get; private set; }
    public string TypeFullName { get; private set; }
    public object Value { get; private set; }

    public NameTypeAndValue(string name, string typeFullName, object value)
    {
        Name = name;
        TypeFullName = typeFullName;
        Value = value;
    }
}

public static class ITypeSymbolExtensions
{
    // Converts names like `string` to `System.String`
    public static string GetTypeFullName(this ITypeSymbol typeSymbol) =>
        typeSymbol.SpecialType == SpecialType.None
        ? typeSymbol.ToDisplayString()
        : typeSymbol.SpecialType.ToString().Replace("_", ".");
}

public static bool TryGetAttributeAndValues(
    this AttributeData attributeData,
    string attributeFullName,
    SemanticModel model,
    out IEnumerable<NameTypeAndValue> attributeValues)
{
    var attributeValuesList = new List<NameTypeAndValue>();
    var constructorParams = attributeData.AttributeConstructor.Parameters;

    // Start with an indexed list of names for mandatory args
    var argumentNames = constructorParams.Select(x => x.Name).ToArray();

    var allArguments = attributeData.ConstructorArguments
        // For unnamed args, we get the name from the array we just made
        .Select((info, index) => new KeyValuePair<string, TypedConstant>(argumentNames[index], info))
        // Then we use name + value from the named values
        .Union(attributeData.NamedArguments.Select(x => new KeyValuePair<string, TypedConstant>(x.Key, x.Value)))
        .Distinct();

    foreach(var argument in allArguments)
    {
        attributeValuesList.Add(
            new NameTypeAndValue(
                name: argument.Key,
                typeFullName: argument.Value.Type.GetTypeFullName(),
                value: argument.Value.Value));
    }
    attributeValues = attributeValuesList.ToArray();
    return true;
}