Dynamically select columns in runtime using entity framework

问题

I have an existing function like this

public int sFunc(string sCol , int iId)
{
    string sSqlQuery = "  select  " + sCol + " from TableName where ID = " +  iId ;
    // Executes query and returns value in column sCol
}

The table has four columns to store integer values and I am reading them separately using above function.

Now I am converting it to Entity Framework .

public int sFunc(string sCol , int iId)
{
     return Convert.ToInt32(TableRepository.Entities.Where(x => x.ID == iId).Select(x => sCol ).FirstOrDefault());
}

but the above function returns an error

input string not in correct format

because it returns the column name itself.

I don't know how to solve this as I am very new to EF.

Any help would be appreciated

Thank you

回答1:

This might help to solve your problem:

public int sFunc(string sCol, int iId)
{
    var _tableRepository = TableRepository.Entities.Where(x => x.ID == iId).Select(e => e).FirstOrDefault();
    if (_tableRepository == null) return 0;

    var _value = _tableRepository.GetType().GetProperties().Where(a => a.Name == sCol).Select(p => p.GetValue(_tableRepository, null)).FirstOrDefault();

    return _value != null ? Convert.ToInt32(_value.ToString()) : 0;
}

This method now work for dynamically input method parameter sCol.

回答2:

You have to try with dynamic LINQ. Details are HERE

回答3:

You can do this:

        var entity = _dbContext.Find(YourEntity, entityKey);
        // Finds column to select or update
        PropertyInfo propertyInfo = entity.GetType().GetProperty("TheColumnVariable");

回答4:

You could use Reflection, something like this (not tested code):

public string sFunc(string sCol , int iId)
{
  var row = TableRepository.Entities.Where(x => x.ID == iId);
  var type = typeof(row);
  var propInfo = type.GetProperty(sCol);

  if (propInfo != null)
  {
    string value = (string)propInfo.GetValue(row);

    return value;
  }

  return null;
}

回答5:

Instead of passing the string column name as a parameter, try passing in a lambda expression, like:

sFunc(x => x.FirstColumnName, rowId);
sFunc(x => x.SecondColumnName, rowId);
...

This will in the end give you intellisense for column names, so you avoid possible errors when column name is mistyped.

More about this here: C# Pass Lambda Expression as Method Parameter

However, if you must keep the same method signature, i.e. to support other/legacy code, then you can try this:

public string sFunc(string sCol , int iId)
{
    return TableRepository.Entities.Where(x => x.ID == iId).Select(x => (string) x.GetType().GetProperty(sCol).GetValue(x)});
}

You might need to adjust this a bit, I didn't have a quick way of testing this.

来源：https://stackoverflow.com/questions/21084916/dynamically-select-columns-in-runtime-using-entity-framework

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sql

ASP.NET

entity-framework

formatexception