一种DataRow转换实体的泛型方法，使用Expression

标题可能不知所云，但是正文更加不知所云。

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调用 方法如下：

var func = GetFunc<DataRow, Entity>();

var entity = func(datarow);

private static Func<TInput, TResult> GetFunc<TInput, TResult>()
{
        TInput datarow = default(TInput);
        TResult entity = default(TResult);
        var row = Expression.Parameter(typeof(DataRow), "dr");
        var assignment = new List<MemberAssignment>();
        var properties = typeof(TResult).GetProperties();
        foreach (var property in properties)
        {
            assignment.Add(MakeMemberAssignment(datarow, entity, property.Name, property.PropertyType, row));
        }
        var memberInit = MakeMemberInitExpression(assignment.ToArray());
        var lambda = Expression.Lambda(memberInit, row);

        return getFunc(datarow, entity, lambda);
}

static Func<TInput, TResult> getFunc<TInput, TResult>(TInput input, TResult result, LambdaExpression exp)
{
        return exp.Compile() as Func<TInput, TResult>;
}

static MemberInitExpression MakeMemberInitExpression(params MemberBinding[] assign)
{
        var newExp = Expression.New(typeof(Contact).GetConstructor(new Type[] { }));
        var init = Expression.MemberInit(newExp, assign);

        return init;
}

static MemberAssignment MakeMemberAssignment<TInput, TResult>(TInput input, TResult result, string field, Type type, ParameterExpression row)
{
        var mi = typeof(DataRowExtensions).GetMethod("Field", new Type[] { typeof(TInput), typeof(string) });
        mi = mi.MakeGenericMethod(type);
        var fieldName = Expression.Constant(field, typeof(string));
        var call = Expression.Call(mi, row, fieldName);
        var memberInfo = typeof(TResult).GetMember(field)[0];
        var assignment = Expression.Bind(memberInfo, call);

        return assignment;
}