Using PredicateBuilder to build query searching across multiple columns of Entity

问题

I have a list of field names. I am trying to build a predicate to look in the fields to see if they contain the search term. I have gone done the path listed in this original question but do not understand how to do a Contains instead of a NotEqual.

string searchTerm = "Fred";    
foreach (var field in FieldNames)
{
    myPredicate= myPredicate.And(m => m.*field*.Contains(searchTerm));                    
}

My code so far:

public static Expression<Func<T, bool>> MultiColumnSearchExpression<T>(string fieldName,string searchValue)
{
    var parameter = Expression.Parameter(typeof(T), "m");
    var fieldAccess = Expression.PropertyOrField(parameter, fieldName);
   //this next line should do a Contains rather then NotEqual but how?
    var body = Expression.NotEqual(fieldAccess, nullValue);

    var expr = Expression.Lambda<Func<T, bool>>(body, parameter);
    return expr;
}

回答1:

So you want to call String.Contains method.

It is a String class instance method with the following signature

public bool Contains(string value)

which can be mapped to the following Expression.Call overload:

public static MethodCallExpression Call(
    Expression instance,
    string methodName,
    Type[] typeArguments,
    params Expression[] arguments
)

And here is how:

public static Expression<Func<T, bool>> ContainsPredicate<T>(string memberName, string searchValue)
{
    var parameter = Expression.Parameter(typeof(T), "m");
    var member = Expression.PropertyOrField(parameter, memberName);
    var body = Expression.Call(
        member,
        "Contains",
        Type.EmptyTypes, // no generic type arguments
        Expression.Constant(searchValue)
    );    
    return Expression.Lambda<Func<T, bool>>(body, parameter);
}

回答2:

Expression.NotEqual is the binary operator !=. Since your Contains call is not a binary operator, there is no direct replacement for this.

Instead, what you need to do is a method call of Contains. So you first need to get the MethodInfo object for the string.Contains function.

It should look like this:

var method = typeof(string).GetMethod("Contains");
var body = Expression.Call(fieldAccess, method, Expression.Constant(searchValue));

来源：https://stackoverflow.com/questions/43685229/using-predicatebuilder-to-build-query-searching-across-multiple-columns-of-entit

标签

entity-framework

expression-trees

predicatebuilder