c# - 'Unable to cast object of type' in linq query -


i have following query using datatable give error: "unable cast object of type 'system.dbnull' type 'system.string'"

var speclist = (from r in dt.asenumerable()             (decimal)r["stdyear"] == 0 && (string)r["speclization"] != null                                 select r["speclization"]).distinct().tolist();

speclization may be:

  • eng
  • null
  • math

how can solve problem? guide me.

for r["speclization"] ,it can null,but before check null or not ,you convert string first, excetption throw： unable cast object of type 'system.dbnull' type 'system.string'

var speclist = (from r in dt.asenumerable()                 r.field<decimal>("stdyear") == 0 && r.field<string?>("speclization").hasvalue                 select r.field<string?>("speclization").value).distinct().tolist();

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