.NET-Community-Toolkit/CommunityToolkit.Mvvm.SourceGenerators/Extensions/SyntaxNodeExtensions.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using Microsoft.CodeAnalysis;

namespace CommunityToolkit.Mvvm.SourceGenerators.Extensions;

/// <summary>
/// Extension methods for the <see cref="SyntaxNode"/> type.
/// </summary>
internal static class SyntaxNodeExtensions
{
    /// <summary>
    /// Checks whether a given <see cref="SyntaxNode"/> represents the first (partial) declaration of a given symbol.
    /// </summary>
    /// <param name="syntaxNode">The input <see cref="SyntaxNode"/> instance.</param>
    /// <param name="symbol">The target <see cref="ISymbol"/> instance to check the syntax declaration for.</param>
    /// <returns>Whether <paramref name="syntaxNode"/> is the first (partial) declaration for <paramref name="symbol"/>.</returns>
    /// <remarks>
    /// This extension can be used to avoid accidentally generating repeated members for types that have multiple partial declarations.
    /// In order to keep this check efficient and without the need to collect all items and build some sort of hashset from them to
    /// remove duplicates, each syntax node is symply compared against the available declaring syntax references for the target symbol.
    /// If the syntax node matches the first syntax reference for the symbol, it is kept, otherwise it is considered a duplicate.
    /// </remarks>
    public static bool IsFirstSyntaxDeclarationForSymbol(this SyntaxNode syntaxNode, ISymbol symbol)
    {
        return
            symbol.DeclaringSyntaxReferences.Length > 0 &&
            symbol.DeclaringSyntaxReferences[0] is SyntaxReference syntaxReference &&
            syntaxReference.SyntaxTree == syntaxNode.SyntaxTree &&
            syntaxReference.Span == syntaxNode.Span;
    }
}