如何使用Express.call方法为从DataTable获取数据创建（谓词）？[英] How to create (predicate) for fetching data from DataTable using Express.call method

问题描述

I have a DataTable in Home Controller as follows:

public DataTable GetTable()
{
    DataTable table = new DataTable();
    table.Columns.Add("Dosage", typeof(int));
    table.Columns.Add("Drug", typeof(string));
    table.Columns.Add("Patient", typeof(Info));
    table.Columns.Add("Date", typeof(DateTime));

    table.Rows.Add(25, "Indocin", new Info("India"), DateTime.Now);
    table.Rows.Add(50, "Enebrel", new Info("UK"), DateTime.Now);
    table.Rows.Add(10, "Hydralazine", new Info("Bhutan"), DateTime.Now);
    table.Rows.Add(21, "Combivent", new Info("India"), DateTime.Now);
    table.Rows.Add(100, "Dilantin", new Info("GreenLand"), DateTime.Now);
    return table;
}

Info class as follows

public class Info
{
    public string Address { get; set; }
    public Info(string Add) {
        this.Address = Add;
    }
}

Now, i want to do the filtering operation based on Address Field, i.e Patient.Address

Here, Patient is object of Info class

I need to form the condition, in order to fetch the data.

I am forming condition using Express.call method

private static MethodCallExpression GetFieldCallExpression(Expression memExp, Type ColumnType, string ColumnName)
        {
            MethodInfo method = typeof(DataRowExtensions).GetMethods().Where(m => m.Name == "Field" && m.IsGenericMethod && m.GetParameters().Count() == 2 && m.GetParameters()[1].ParameterType == typeof(string)).FirstOrDefault();
            var genericWrapper = method.MakeGenericMethod(new Type[] { ColumnType });
            var toLowerMethodCall = Expression.Call(
                 null,
                 genericWrapper,
                 memExp,
                 Expression.Constant(ColumnName, ColumnName.GetType())
                 );
            return toLowerMethodCall;
        }

Here, memExp - instance of DataRow columnName - Patient.Address columnType - string

Predicate is formed as like this

It got the message that,

"Column 'Patient.Address' does not belong to table ."

Where did i commit my mistake

推荐答案

DataRow value accessors support only the "top level" column names (like "Dosage", "Drug", "Patient" etc. in your sample). In order to achieve your goal, you have to split the path and generate accessors manually.

For instance, the sample accessor for "Patient.Address" should be dr.Field<Info>("Patient").Address or in other words:

var value1 = dr.Field<Info>("Patient");
var value2 = value1.Address;
...
return value2;

Here is the same implemented generically:

private static Expression GetFieldValueExpression(Expression source, Type columnType, string memberPath)
{
    var memberNames = memberPath.Split('.');
    Expression value = Expression.Call(
        typeof(DataRowExtensions), "Field", new Type[] { columnType },
        source, Expression.Constant(memberNames[0]));
    for (int i = 1; i < memberNames.Length; i++)
        value = Expression.PropertyOrField(value, memberNames[i]);
    return value;
}