ExpressionExtensions.cs

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using System;
using System.Linq.Expressions;
 
using Tgstation.Server.Host.Authority.Core;
 
namespace Tgstation.Server.Host.Extensions
{
    static class ExpressionExtensions
    {
        public static Expression<Func<TQueried, ProjectedPair<TQueried, TResult>>> Projected<TQueried, TResult>(this Expression<Func<TQueried, TResult>> translationExpression)
        {
            var parameter = Expression.Parameter(typeof(TQueried), "queried");
            var body = Expression.Invoke(translationExpression, parameter);
 
            var ourType = typeof(ProjectedPair<TQueried, TResult>);
 
            var expr = Expression.MemberInit(
                Expression.New(ourType),
                Expression.Bind(
                    ourType.GetProperty(nameof(ProjectedPair<TQueried, TResult>.Queried))!,
                    parameter),
                Expression.Bind(
                    ourType.GetProperty(nameof(ProjectedPair<TQueried, TResult>.Result))!,
                    body));
 
            return Expression.Lambda<Func<TQueried, ProjectedPair<TQueried, TResult>>>(expr, parameter);
        }
    }
}