InterfaceGenerator/InterfaceGenerator/SymbolExtensions.cs

using System.Collections.Generic;
using System.Linq;
using Microsoft.CodeAnalysis;

namespace InterfaceGenerator
{
    internal static class SymbolExtensions
    {
        public static bool TryGetAttribute(
            this ISymbol symbol,
            INamedTypeSymbol attributeType,
            out IEnumerable<AttributeData> attributes)
        {
            attributes = symbol.GetAttributes()
                               .Where(a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, attributeType));
            return attributes.Any();
        }

        public static bool HasAttribute(this ISymbol symbol, INamedTypeSymbol attributeType)
        {
            return symbol.GetAttributes()
                         .Any(a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, attributeType));
        }
    }
}