如何在返回Expression的C#方法中调用返回Func<>的方法

public static System.Linq.Expressions.Expression<Func<MetaMovie, bool>> IsEqual(MetaMovie other)
{
    if (other.Id > 0) return m => other.Id == m.Id;

    return m => other.MetaSource == m.MetaSource && other.ExternalId == m.ExternalId;
}

调用上述方法后变量的DebugView内容：

.Lambda #Lambda1<System.Func`2[com.cyberinternauts.all.MediaRecognizer.Models.Metas.MetaMovie,System.Boolean]>(com.cyberinternauts.all.MediaRecognizer.Models.Metas.MetaMovie $m)
{
    (.Constant<com.cyberinternauts.all.MediaRecognizer.Models.Metas.MetaMovie+<>c__DisplayClass81_0>(com.cyberinternauts.all.MediaRecognizer.Models.Metas.MetaMovie+<>c__DisplayClass81_0>).other).MetaSource ==
    $m.MetaSource && (.Constant<com.cyberinternauts.all.MediaRecognizer.Models.Metas.MetaMovie+<>c__DisplayClass81_0>(com.cyberinternauts.all.MediaRecognizer.Models.Metas.MetaMovie+<>c__DisplayClass81_0>).other).ExternalId ==
    $m.ExternalId
}

希望创建一个Func<MetaMovie, bool> GetComparison(MetaMovie other)方法，供IsEqual方法调用，但无法成功集成。

public static Func<MetaMovie, bool> GetComparison(MetaMovie other)
{
    if (other.Id > 0) return m => other.Id == m.Id;

    return m => other.MetaSource == m.MetaSource && other.ExternalId == m.ExternalId;
}

public static Expression<Func<MetaMovie, bool>> IsEqual(MetaMovie other)
{
    var param1 = Expression.Parameter(other.GetType(), "param1");

    return Expression.Lambda<Func<MetaMovie, bool>>(Expression.Call(GetComparison(other).Method), param1);
}

IsEqual方法的return行报错：
Static method requires null instance, non-static method requires non-null instance

错误原因与解决方法 #

错误本质是：你试图直接调用匿名委托的Method属性，但这些匿名方法是绑定到捕获变量的显示类实例的实例方法，调用时必须传入对应的实例作为第一个参数；更关键的是，这种方式会破坏表达式树的可解析性（比如EF无法翻译该表达式执行数据库查询）。

方案一：让GetComparison返回表达式树（推荐） #

直接修改GetComparison返回表达式树，这样IsEqual可以直接复用其结果，完全保留原有逻辑的表达式树特性：

public static Expression<Func<MetaMovie, bool>> GetComparison(MetaMovie other)
{
    if (other.Id > 0) 
        return m => other.Id == m.Id;

    return m => other.MetaSource == m.MetaSource && other.ExternalId == m.ExternalId;
}

public static Expression<Func<MetaMovie, bool>> IsEqual(MetaMovie other)
{
    return GetComparison(other);
}

方案二：必须返回委托时，手动构建表达式树 #

如果业务限制必须让GetComparison返回编译后的委托，那么需要在IsEqual中手动构建等价的表达式树，避免直接调用委托方法：

public static Func<MetaMovie, bool> GetComparison(MetaMovie other)
{
    if (other.Id > 0) return m => other.Id == m.Id;

    return m => other.MetaSource == m.MetaSource && other.ExternalId == m.ExternalId;
}

public static Expression<Func<MetaMovie, bool>> IsEqual(MetaMovie other)
{
    var param = Expression.Parameter(typeof(MetaMovie), "m");
    Expression body;

    if (other.Id > 0)
    {
        var otherId = Expression.Constant(other.Id);
        var paramId = Expression.Property(param, nameof(MetaMovie.Id));
        body = Expression.Equal(otherId, paramId);
    }
    else
    {
        var otherMetaSource = Expression.Constant(other.MetaSource);
        var paramMetaSource = Expression.Property(param, nameof(MetaMovie.MetaSource));
        var sourceEqual = Expression.Equal(otherMetaSource, paramMetaSource);

        var otherExternalId = Expression.Constant(other.ExternalId);
        var paramExternalId = Expression.Property(param, nameof(MetaMovie.ExternalId));
        var idEqual = Expression.Equal(otherExternalId, paramExternalId);

        body = Expression.AndAlso(sourceEqual, idEqual);
    }

    return Expression.Lambda<Func<MetaMovie, bool>>(body, param);
}

内容的提问来源于stack exchange，提问作者Master DJon